Air speed, or lateral air movement, is the maximum rate at which a character can move left or right while in midair. Air speed, air acceleration, falling speed, and gravity are factors for how far a character can recover.
Airspeed is often confused with how far a character can travel without landing. Characters with low falling speeds tend to be able to travel farther in the air, simply due to hitting the ground later, not due to traveling through the air faster. As a result, characters with higher falling speeds are often perceived as having lower airspeeds, when in reality, some fast falling characters can move around in the air faster than the floaty characters.
Super Smash Bros. air speed ranking