An attack or tactic designed to thwart challengers who attack from the sky. Between up tilt and up smash, a fighter usually has a quicker and slower obvious anti-air option (though certainly this isn’t global, and plenty of up tilts and up smashes have distinguishing quirks). Sometimes a fighter’s up special fits the bill too, though usually this is pretty risky since most of them have lots of lag afterward. Quicker up air attacks also fit the role.
Often the best anti-air strategy is to take to the sky first, walling off the airspace by pre-emptively jumping or hopping using good air moves with long active periods. Ideal air moves for this have low enough lag to be repeated several times per jump and double jump, overwhelming fighters saddled with slower air arsenals.