Plain English Meaning
The airplane's speed through the air is changing fast enough that you can see the airspeed needle moving noticeably, not just drifting slowly.
Full Definition
A sudden increase or decrease in the speed of the aircraft through the air, occurring quickly enough to be felt by the pilot and reflected by an immediate movement of the airspeed indicator needle. In instrument flying, such a change is treated as a flight cue indicating an unintended pitch or power deviation that requires correction.
Context Anchor
Seen in airspeed indicator discussions, especially when learning how the instrument responds while the aircraft is accelerating, slowing down, or changing .