A trap hitbox is one kind of hitbox.
When it comes into contact with an opponent’s strike hitbox, a reaction occurs, and the resulting action completes the trap move.
In Japanese fighting game terminology, what English gamers know as a “trap” in the sense of a trap hitbox (which uses an opponent’s strike attempt against him) is called “atemi”.
The original meaning derives from Japanese martial arts terminology, where “atemi” refers to strikes that aim for vital points, or just strikes. However, in Gamest magazine, where they abbreviated Geese Howard’s “Atemi Nage” (the first trap move, from Fatal Fury) as “atemi”, the term stuck (though other terms have been attempted to avoid confusion, they aren’t as popular).