In a game or sport, what must be accomplished for the winner to be determined. In soccer, for example, the victory condition is to have more goals scored than the other team when the game's time has run out.
On the other hand, other games have more complicated victory conditions. In chess, for example, you must checkmate your opponent.
In an extreme example, the Babylon 5 and Illuminati collectible card games have many cards which set victory conditions, so few games will end in the same way!
Many games' sole victory condition is that all other players be eliminated from the game, which would occur when a Defeat Condition has been met.
Some games have multiple layers of victory in their conditions, such as Old Maid, in which the first player to run out of cards is the Grand Winner, while all others who eventually run out of cards are winners, and the last is the Old Maid.