The way I see it, there's a distinction between "normal" XP gain and grinding.

Normal XP is a reflection (however artificial) of your progress in developing your character. Normally it's a bar that expands as you complete quests, kill mobs, find new zones, or whatever mechanic the developer uses to measure "personal growth."

That's something that - in a good game - happens as you play, be it that you're fighting mobs, winning pvp battles, crafting or completing quests. In some way, some progress is made on some internal counter somewhere, but it happens as a by-product of your activities when they're judged to be worthy of recognition by the game.

To me, grinding on the other hand is when a player ignores the "what" and focuses on the "why" of the thing. In this case, rather than play the game and advance as they do so, they choose to make advancement - usually through a repetitive cycle of actions - their sole focus. It's - as I say - about doing a calculation, watching a bar move and then repeating the process simply for the sake of reaching the next level, at which point it starts over.

I know it's a minor distinction in some games (as I say, EQ was totally misnamed, as you rarely "Quested" and day-to-day advancement only came through mob grinding) but as I say, in a good game, you'll - or rather you SHOULD - advance/level up in a way that's a side-effect of just playing the game.