The amount of experience an individual Pokémon has is an indication of how much it has battled. It is quantified as Experience Points, which a Pokémon can gain in battle by defeating an opponent without fainting first. After a certain amount of experience points have been gained, a Pokémon will grow a level, all the way up to level 100, where a Pokémon will no longer gain experience (in the first two generations, the game will still erroneously state that it has gained experience points).
Experience is normally gained by all Pokémon who have been sent out against an opponent's Pokémon, divided evenly among them. Experience is gained upon the opponent Pokémon fainting, and its amount is calculated as a function of the fainted Pokémon's level, as well as species. Certain items can affect the distribution and amount of experience gained, as can other conditions, such as whether or not the Pokémon was caught by another person or in another language of the game.