A perfect definition of roleplaying has the following features:
1. It includes everything that is roleplaying.
2. It includes nothing that is not roleplaying.
3. It uses no jargon.
4. It does not refer to “roleplaying games”, or provides a noncircular and perfect definition for them, too. Ditto story-games and other closely related concepts.
My definition fails on the second criterion, IMO. I still consider it a fairly good one, if not the shortest possible.
An activity is roleplaying if and only if all the following criteria apply to it.
1. It is social in that there are at least two persons whose activity qualifies by the other two criteria. (If an alien, a sufficiently advanced AI, or a supernatural but intelligent entitity happen to roleplay, they qualify, too.)
2. The aforementioned participants make decisions for characters. (Thinking in character is sufficient but not necessary.)
3. The aforementioned decisions matter in the fiction of the game in a way that the players care about.
That’s it. Most, if not all, forms of miniature/war/figure gaming are included, which is the major flaw of the definition. Or just an observation on the similarity of such games and rpgs. Whichever way one wants to take it.