Terminal nodes are leaf nodes with a fixed, application-dependent value (e.g., win, loss, draw). Thus, nodes representing mate or stalemate positions, which have per game definition no more child nodes, since there are no legal moves. If the root becomes a terminal node, the game is finished. That virtually also applies for three-fold repetitions, fifty-move rule and draws by insufficient material, despite strict interpretation of the rules requires a claim and an arbiter instance to finally declare them terminal.
In a wider sense, the search below the root may consider nodes where a cycle occurred (repetition but not yet necessarily three-fold) terminal as well, similar for nodes, where interior node recognizers detect a node state (e.g., a table base draw) with no need to search it further.