A win condition is the criterion that decides when a game ends and who (if anyone) has won. Checkmate in chess. The most points when the deck runs out. Survival when all other players are eliminated. The win condition gives direction to every decision a player makes — it is what makes choices strategic rather than arbitrary.
Not all games have win conditions in the competitive sense. Cooperative games define success collectively (the group wins or loses together). Sandbox and narrative games like many role-playing games may have no formal win condition at all — the objective is to create a satisfying shared experience rather than to defeat other participants. Some games have multiple win conditions (first to reach X points or control Y territories), which creates strategic branching as players pursue different paths.
The relationship between win condition and mechanics shapes the texture of play. A game where you win by accumulating points encourages optimization. A game where you win by elimination encourages aggression. A game where you win by consensus (as in Brave Old World’s conflict resolution) encourages negotiation. Changing the win condition while keeping the same mechanics can produce a different game.