The dangerous, damage-dealing area of a move. When you talk about the active period, it concerns how fast-hitting and long-lasting a move is; when you talk about the hitbox, it’s how much area it covers. Usually attacks strike where it looks like they should: a grab pulls people in around the extended hand, a punch generally hits at the fist and forearm, a sword swing hits around the sword and perhaps the arm, and so on. Occasionally the visuals don’t tell the full story or are misleading, which this guide’s move descriptions highlight (like for a forward-facing headbutt that has an exceptional hitbox to the back early, or a move that looks like it arcs through an upward direction but actually only hits horizontally).
Contrast with hurtbox, the vulnerable parts of a fighter. Hitboxes are sometimes described as disjointed when they extend far beyond the hurtbox, a great trait to have.