Ath'kethin
Elder Thing
Novels aren't canon 

Someone could probably find an announcement or something from wotc explaining it, but it's a split from the old ways. In the past the d&d rulebooks were written in a style that could be described as a form of technical writing. With 5e they made it a point to avoid the added difficulty of technical writing & just write in a grammatically correct style that is often unclear confusing or downright useless for conveying technical things like the rules for a relatively complex exception based ruleset.What is Natural Language?
Someone could probably find an announcement or something from wotc explaining it, but it's a split from the old ways. In the past the d&d rulebooks were written in a style that could be described as a form of technical writing. With 5e they made it a point to avoid the added difficulty of technical writing & just write in a grammatically correct style that is often unclear confusing or downright useless for conveying technical things like the rules for a relatively complex exception based ruleset.
TL;DR it's a bad excuse
That came around in 4e, so it's not a "specifically 5e" turn of phrase."Opportunity Attack" vs "Attack of Opportunity." I find the former easier to say, so I prefer it.
Seconded. Especially rare for something not found in any rulebook.Bounded accuracy
I guess these are phrases, plus "Lair Actions", but it sounds more like keywords. Sorry, I'm being pendantic - to me these are more names of mechanics, which of course could be unique to an edition of rules, than a "turn of phrase". I was originally thinking "per short rest" but discarded it as just mechanics, not a phrase. I dunno. Probably it counts and it's me.Concentration spells. Legendary Actions. Legendary Resistance. Bonus Actions.
Ah. I did not play 4e so that slipped by me.That came around in 4e, so it's not a "specifically 5e" turn of phrase.
Yeah, I thought that game keywords/names of unique mechanics fall into a gray area, too, but I posted them anyway. I guess we would need more guidance from @Shades of Eternity on what they are looking for.I guess these are phrases, plus "Lair Actions", but it sounds more like keywords. Sorry, I'm being pendantic - to me these are more names of mechanics, which of course could be unique to an edition of rules, than a "turn of phrase". I was originally thinking "per short rest" but discarded it as just mechanics, not a phrase. I dunno. Probably it counts and it's me.![]()