Plain English Meaning
Quick up or down pushes of air that hit the airplane and briefly make the wings carry more or less weight than usual.
Full Definition
Sudden vertical movements of air, either upward or downward, that strike an aircraft in flight and momentarily change its angle of attack, producing an abrupt increase or decrease in load factor.
Context Anchor
Seen in load factor, turbulence, and aircraft handling discussions, especially when explaining why rough air can increase stress on the airplane.
Derivation
Gust' comes from the Old Norse 'gustr', meaning a sudden rush of wind. 'Vertical' simply specifies that the rush is up or down rather than horizontal — which matters because vertical gusts directly change how hard the wings are working.