Air speed, or lateral movement, is the rate at which a character can move left or right while in midair. Air speed, falling speed and gravity are all factors in how far a character can recover.
Air speed 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 travelling through the air faster. As a result, characters with higher falling speeds are often perceived as having lower air speeds.