I use a very technically applicable definition, for myself. I have a spectrum of Role–Rheme which I use for classifying gameworlds and such. Again, probably more useful for game design than for game play.
Anyways . . .
A ha! Thanks be to you, Illysia; I just now discovered how to describe the differences of purpose between me and most of the people here who call themselves role–players — and, indeed, many enclaves of role–players anywhere.
You see, for me, the appeal of taking an avatar lies in the immersive aspects. Stories result upon the interaction between the setting of that player and my use of the player, and are certainly a perk, but not the sole justification for the engagement; well, here I especially refer to the balladic aspect of stories, and less so the mythic aspect.
Compare that to most of the role–playing done here, and by those who hail or take influence from the dice–rolling & pen–and–paper heritage, which emphasizes creating actors for use in stories and adventures. Leastways, that's how it appears to me.
I hope that elucidates my post
here regarding Plotting and Pantsing.