Exploration - finding out stuff
Combat - beating people/things.
Social - interacting with people. This typically involves elements of finding out stuff, beating people, but also mutually beneficial interaction - alliance building, mate-bonding, friendship et al.
To be honest, even in the context of 5e D&D this is looking to me like a dubious system of classification in the way that you present it, because social can include both exploration and combat.
To me, it seems fairly clear that the utility of the "3 pillars" is not in identifying categories of fictinal activity, but in identifying ways of handling stuff at the table. In particular, as I understand it, the 5e "3 pillars" are an attempt to capture in a single classificatory scheme some D&D traditions concerning how different sorts of action-resolution are handled.
Combat pertains to the sort of stuff that is traditionally resolved using the to hit, saving throw and hit point mechanics. For these reasons, some but not all spell casting falls under the combat pillar.
Exploration pertains to the sort of stuff that is traditionally resolved using classic dungeoneering and hex-crawl adjudication, which includes: answering questions by reference to the referee's map and key; rolling dice for search attempts, listening at doors, and avoiding getting lost; and the GM describing new situations as the PCs move into them.
Social pertains to PC-NPC interactions, which many tables traditionally resolve by free roleplaying.
It's fairly clear that the boundaries of these pillars, even within a very traditional D&D paradigm, are highly porous. When
exploration results in the PCs triggering a trap, for instance,
combat processes might be used to determine the result (eg a saving throw to avoid damage). When NPCs are encountered, the use of a random reaction roll may help determine the outcome of what occurs in subsequent
social interaction, but as a method of resolution reallly has more in common with the methods of the
exploration pillar.
Then there is a case like determining surprise: does this belong to
exploration or to
combat? I don't see that there is anything meaningful at stake in this question - it's "angels on the head of a pin" territory.
And there are also elements of traditional D&D that seem to me to fall under none of the pillars - eg classic D&D wilderness evasion (based on a % chance rolled periodcially modified by various factors - charts for this are found in AD&D and Marsh/Cook Expert, and maybe in OD&D too though I'm not pulling out my books to check at the moment). This is not social because not free roelplaying of the traditional sort. It's not exploration. But it's not combat in the traditional sense either, in that it doesn't reference any of the standard combat notions like to hit chance, damage, etc. Again, insisting that this particular mechanic
must fall under one of the three pillars seems to me to add nothing to our understanding of D&D play, nor to be useful in thinking about how to GM or play the game.
It follows from what I've just said, and frankly I think should be pretty uncontroversial, that a system that doesn't follow these traditional D&D understandings of how certain sorts of actions are adjudicated isn't usefully thought of in terms of the "3 pillars". So in 4e, for instance, if social encounters are being resolved as skill challenges, and if searching for stuff and avoiding getting lost is being resolved as skill challenges, then GMs and players need good advice on how to adapt the skill challenge mechanics to this variety of stuff, but no clarity in analysis or advice is gained by distinguishing "social" from "exploration". (And the odd-one-out category of classic evasion would also, in 4e, be just another thing to be resolved as a skill challenge. Likewise building and repairing stuff, although depending on details this can suffer a bit from a lack of clarity in respect of salient skills - in my game Dungeoneering ends up picking up a fair bit of this slack.)
In BW, there is a social conflict resolution system called Duel of Wits that, in mechanical framing, is very similar to the combat resolution mechanics. And the combat resolution mechanics distinguish between melee (Fight!) and missile skirmishing (Range and Cover). So if one wants to talk about "pillars" here at all, then skirmish combat is as much a distinct pillar from melee combat as either is from social conflict. But all three have more in common than the 5e combat and social pillars, as all three are based on a common framework of blind declaration within a quasi-rocks/paper/scissors opposed check framework.
None of the above is a criticism of 5e's treatment of its pillars. But it is a criticism of the idea that these provide any general framework for thinking about RPGing.
Travelling a known route isn't Exploration, but travelling an unknown route & encountering new places along the way does have an Exploration element I think, in 5e D&D terms. Of course if it's abstracted - line on the map style - then it's not Exploration, it's closer to Downtime, as Mr P noted.
If travelling a known route in 5e isn't exploration - and clearly it isn't combat or social - then we have another activity which falls outside the 3 pillars. Which is fine as far as it goes, but if such examples proliferate then the utility of the classification becomes more doubtful.
What is interesting to me is that, if it
isn't exploration, then that suggets it's not meant to be a big element of play at all, and should be handled line-on-a-map/"downtime" style until something occurs (whether by way of player action declaration or by way of GM stipulation) that does genuinely fall under one of the pillars. I wonder if this is the standard way of playing 5e, though, or if some (many?) tables do treat travelling known routs as exploration.
I think that travelling an unknown route should count as exploration in 5e D&D, because of the resolution systems that should activate: both the players asking questions and getting GM answers, plus the stuff like spotting things, avoiding getting lost, etc that I identified above as traditional components of D&D's "exploration" pillar.
And we are likeminded in thinking that in an RPG where travel even along new routes is handed simply through fairly quick free narration, with none of those traditional D&D methods being deployed, then we don't have something that counts as 5e-type exploration. This describes most of the travel in my Prince Valiant game, all of the travel in my Marvel Heroic game, most fo the travel in my Cortex+ Heroic Fantasy game, some of the travel in my BW game (ie if I've "said 'yes'" rather than framed a check), and interstellar travel in my Classic Traveller game.
