Terminal positions are where the game ends. No more moves are possible, and the outcome is determined by the game rules. In some games, reaching a terminal position means you win. In others, getting stuck means you lose. The rules vary by problem. Identifying terminal positions is the first step in game analysis.
These are your base cases when working backward through the game tree. For example, in a game where you capture all opponent pieces, a terminal position is when no pieces remain. In a graph traversal game, reaching a specific target node ends the game.