Another thread came up with the intriguing idea that D&D died when TSR died:
www.enworld.org
As always, things go well when people mark some editions as "D&D" and other editions as "Not D&D."
Notably, @MichaelSomething made the following observation-
"First thing is, what is D&D? We can't even seem to reach a common agreement on what that is."
Which is a great point! How can we even get our blood boiling by screaming about what isn't D&D, until we at least define what is D&D? In order to assist this uncontroversial effort, I thought I might channel my inner Wittgenstein .... "D&D, D&D, what is D&D? That D&D that we cannot define, we must pass over in silence."
Now, here is my stab at a working definition of "D&D":
A. My Head Was Screaming No, But My Mouth Was Chewing Gum
The Ship of Theseus, eh? We are all familiar with it, but if you aren't, a quick primer- Theseus's ship is kept in the harbor. Over time, as the ship rots, the old rotted wooden parts are replaced by new parts. Eventually, all the parts .... the entire ship .... has been replaced. Is the restored ship the original ship? Is the ship that is now in the harbor, the Ship of Theseus?
There are variations on this, and of course there is the whole, "What if every rotted part was put in a warehouse, and then at some later date, was cured of rot, and all of the original pieces were then re-assembled ... what then? And what if the two Ships of Theseus then got into a transporter accident and merged somehow, thus teaching us a valuable lesson about racism?" Admittedly, I often confuse my philosophy with old Star Trek episodes.
It's an interesting thought experiment about identity, and it has some limited value when thinking about the continuity of D&D over time; as editions continue over time, they add, replace, and subtract material. As a new edition replaces an old edition, there are often extensive re-workings of old and rotted rules (ahem). At what point (if any) do the alterations become too extensive? At what point are you no longer playing the "same" D&D as Theseus?
Of course, this thought exercise has a limited utility when it comes to brands and IP like D&D. The rules and lore don't rot, and they aren't replaced by exact replicas. Instead, they change and evolve over time. A better analogy would be to look at examples like cars and shared fictional universes.
B. This Certainly is an Upsetting Number of Pancakes
What is a car? Four wheels, etc. But more importantly, what is a mustang? A corvette? A bronco? A wrangler? A land cruiser? A 911?
All of those are models of car that have span years (in many cases, decades .... the 911 since 1963), and most of them are identifiable in certain ways. Car technology has changed greatly over time, but there is some commonality in the lineage that ties certain models together. If you're a car person, you know exactly what that means.
But if you're not, think of the more recent concept of the "shared universe." Whether it's the Buffyverse, or the MCU, or Doctor Who, or Star Trek, or Star Wars, there is a set of associated "signals" and "branding" and "IP" that let us know that something exists within that universe. Regardless of quality (which is hotly debated by fans!) or timelines or canon or any other factors, we know that, for example, the new Below Decks and the Chris Pine movies and Discovery and ST:TNG and ST:TOS are all Star Trek.
It's the same with characters; Batman has been portrayed countless ways and times; from camp to grimdark, from brawny thug to brainy detective, to everything in between. Yet they are all "Batman". So I think looking at this concept provides us with the first glimmer as to a working meaning of "D&D." I'm going to paraphrase someone again-
For a large class of cases in which we employ the word D&D it can be defined thus: the meaning of D&D is its use in the language. And the primary use of D&D in the language is a descriptor for the TTRPG brand that is specific to TSR/WoTC/Hasbro.
So D&D, at it's core, is the OD&D/1e/2e/3e/4e/5e and BX/BECMI/RC line of games. Or, put another way, D&D are the games designated as such by the D&D rights holder.
Kind of unsatisfying, isn't it? But wait, there's more!
C. We don’t go home. We’re burned to death up there. Then the company wakes up a new one. Every three years. Like clock work.
While it might seem that the prior definition is both overly expansive and unsatisfying (art is what artists do), it is not expansive enough. Because I would go further; D&D isn't just "official" D&D, but D&D in terms of usage, now encompasses a whole style of play and rules. This gets a little more interesting. IMO.
Quick link to terms, if you aren't familiar:
www.enworld.org
Going back to the Ship of Theseus, again, there is an entire universe of games that emulate D&D; they are clones of the rules. Maybe they have a tweak here or there, but these games, everything from OSE to OSRIC to so much more (retroclones) are, in fact, D&D. Any game that seeks to emulate and/or clone the rules in whole or in part to simulate a version of D&D is D&D.
...and that would include Pathfinder. That means that Pathfinder is D&D.
So this brings up three controversial edge cases, all of which I would say are not D&D.
1. What if it isn't fantasy? There were a number of d20 games that are not fantasy. The D&D rules in different editions have been adopted for use in different non-fantasy campaigns. There is noting wrong with playing a non-fantasy campaign, and there is nothing wrong with using "D&D rules" for something that isn't fantasy. However, by using the term "D&D," (which, before we forget, means Dungeons & Dragons) you are necessarily invoking fantasy. As such, while you can look at the amount of "fantasy" necessary, I do not think you can use "D&D" to refer to, for example, a purely modern or science fiction campaign.
2. What if it's based on a clone, but not based on D&D? This is the "PF2" exception. PF can be called D&D; but PF2 was developed independently, with new rules, regarding PF. So PF2 is not D&D, even though PF is.
3. What if it's not designed to emulate the rules, but the "feel" of D&D? If a game is using a completely different underlying engine (PbTA, BiTD), but is looking to emulate the feel of a classic fantasy dungeon game ... it's not D&D. It might be a great game, but unless it is trying to emulate the rules of one of the D&D games, it isn't "D&D."
Now, if you read all of that, you might be thinking, "Okay, what about AIME (compatible with 5e)? What's the status of that?" To which I reply ...."This is why we have comments after the OP."
D. If you want a happy ending, it depends on where you stop the story.
Why do all of this? Because it's there. Discussing what is, and isn't D&D (especially when it comes to actual editions) seems like a fool's errand; defining something as "not D&D" is often just a way to get personal preferences enshrined as fact, and often happens in these types of discussions (Well, that movie isn't really Star Wars! That show isn't really Star Trek! That Doctor isn't really a Gallifreyan!).
Based on what I wrote, I'd go with the following definition of D&D:
Those fantasy TTRPGs designated as "D&D" by the rights holder, and those fantasy TTRPGs that seek to emulate the official "D&D" games by cloning the rules in whole or in part.
Now, what does everyone else think?

D&D General - Did D&D Die with TSR?
I was going to post this in one of the celebratory threads about the 20th anniversary of 3rd edition, and decided that it was too tangential and thread-cappy to go there. So I am putting it in its own thread so we can discuss (if you feel like it). I'm not an expert historian about TTRPGs and...

As always, things go well when people mark some editions as "D&D" and other editions as "Not D&D."

Notably, @MichaelSomething made the following observation-
"First thing is, what is D&D? We can't even seem to reach a common agreement on what that is."
Which is a great point! How can we even get our blood boiling by screaming about what isn't D&D, until we at least define what is D&D? In order to assist this uncontroversial effort, I thought I might channel my inner Wittgenstein .... "D&D, D&D, what is D&D? That D&D that we cannot define, we must pass over in silence."
Now, here is my stab at a working definition of "D&D":
A. My Head Was Screaming No, But My Mouth Was Chewing Gum
The Ship of Theseus, eh? We are all familiar with it, but if you aren't, a quick primer- Theseus's ship is kept in the harbor. Over time, as the ship rots, the old rotted wooden parts are replaced by new parts. Eventually, all the parts .... the entire ship .... has been replaced. Is the restored ship the original ship? Is the ship that is now in the harbor, the Ship of Theseus?
There are variations on this, and of course there is the whole, "What if every rotted part was put in a warehouse, and then at some later date, was cured of rot, and all of the original pieces were then re-assembled ... what then? And what if the two Ships of Theseus then got into a transporter accident and merged somehow, thus teaching us a valuable lesson about racism?" Admittedly, I often confuse my philosophy with old Star Trek episodes.
It's an interesting thought experiment about identity, and it has some limited value when thinking about the continuity of D&D over time; as editions continue over time, they add, replace, and subtract material. As a new edition replaces an old edition, there are often extensive re-workings of old and rotted rules (ahem). At what point (if any) do the alterations become too extensive? At what point are you no longer playing the "same" D&D as Theseus?
Of course, this thought exercise has a limited utility when it comes to brands and IP like D&D. The rules and lore don't rot, and they aren't replaced by exact replicas. Instead, they change and evolve over time. A better analogy would be to look at examples like cars and shared fictional universes.
B. This Certainly is an Upsetting Number of Pancakes
What is a car? Four wheels, etc. But more importantly, what is a mustang? A corvette? A bronco? A wrangler? A land cruiser? A 911?
All of those are models of car that have span years (in many cases, decades .... the 911 since 1963), and most of them are identifiable in certain ways. Car technology has changed greatly over time, but there is some commonality in the lineage that ties certain models together. If you're a car person, you know exactly what that means.
But if you're not, think of the more recent concept of the "shared universe." Whether it's the Buffyverse, or the MCU, or Doctor Who, or Star Trek, or Star Wars, there is a set of associated "signals" and "branding" and "IP" that let us know that something exists within that universe. Regardless of quality (which is hotly debated by fans!) or timelines or canon or any other factors, we know that, for example, the new Below Decks and the Chris Pine movies and Discovery and ST:TNG and ST:TOS are all Star Trek.
It's the same with characters; Batman has been portrayed countless ways and times; from camp to grimdark, from brawny thug to brainy detective, to everything in between. Yet they are all "Batman". So I think looking at this concept provides us with the first glimmer as to a working meaning of "D&D." I'm going to paraphrase someone again-
For a large class of cases in which we employ the word D&D it can be defined thus: the meaning of D&D is its use in the language. And the primary use of D&D in the language is a descriptor for the TTRPG brand that is specific to TSR/WoTC/Hasbro.
So D&D, at it's core, is the OD&D/1e/2e/3e/4e/5e and BX/BECMI/RC line of games. Or, put another way, D&D are the games designated as such by the D&D rights holder.
Kind of unsatisfying, isn't it? But wait, there's more!
C. We don’t go home. We’re burned to death up there. Then the company wakes up a new one. Every three years. Like clock work.
While it might seem that the prior definition is both overly expansive and unsatisfying (art is what artists do), it is not expansive enough. Because I would go further; D&D isn't just "official" D&D, but D&D in terms of usage, now encompasses a whole style of play and rules. This gets a little more interesting. IMO.
Quick link to terms, if you aren't familiar:

D&D General - A Brief Glossary of Acronyms and D&D Terms
So on another thread, some people asked "What is B/X?" And "What is OSE?" Other than the joke-y answers, which are great, I thought I'd post a quick reference for those people who see the terms bandied about but where confused as to what all the various acronyms mean. This is not meant to be...

Going back to the Ship of Theseus, again, there is an entire universe of games that emulate D&D; they are clones of the rules. Maybe they have a tweak here or there, but these games, everything from OSE to OSRIC to so much more (retroclones) are, in fact, D&D. Any game that seeks to emulate and/or clone the rules in whole or in part to simulate a version of D&D is D&D.
...and that would include Pathfinder. That means that Pathfinder is D&D.
So this brings up three controversial edge cases, all of which I would say are not D&D.
1. What if it isn't fantasy? There were a number of d20 games that are not fantasy. The D&D rules in different editions have been adopted for use in different non-fantasy campaigns. There is noting wrong with playing a non-fantasy campaign, and there is nothing wrong with using "D&D rules" for something that isn't fantasy. However, by using the term "D&D," (which, before we forget, means Dungeons & Dragons) you are necessarily invoking fantasy. As such, while you can look at the amount of "fantasy" necessary, I do not think you can use "D&D" to refer to, for example, a purely modern or science fiction campaign.
2. What if it's based on a clone, but not based on D&D? This is the "PF2" exception. PF can be called D&D; but PF2 was developed independently, with new rules, regarding PF. So PF2 is not D&D, even though PF is.
3. What if it's not designed to emulate the rules, but the "feel" of D&D? If a game is using a completely different underlying engine (PbTA, BiTD), but is looking to emulate the feel of a classic fantasy dungeon game ... it's not D&D. It might be a great game, but unless it is trying to emulate the rules of one of the D&D games, it isn't "D&D."
Now, if you read all of that, you might be thinking, "Okay, what about AIME (compatible with 5e)? What's the status of that?" To which I reply ...."This is why we have comments after the OP."
D. If you want a happy ending, it depends on where you stop the story.
Why do all of this? Because it's there. Discussing what is, and isn't D&D (especially when it comes to actual editions) seems like a fool's errand; defining something as "not D&D" is often just a way to get personal preferences enshrined as fact, and often happens in these types of discussions (Well, that movie isn't really Star Wars! That show isn't really Star Trek! That Doctor isn't really a Gallifreyan!).
Based on what I wrote, I'd go with the following definition of D&D:
Those fantasy TTRPGs designated as "D&D" by the rights holder, and those fantasy TTRPGs that seek to emulate the official "D&D" games by cloning the rules in whole or in part.
Now, what does everyone else think?