A phenomenon first described by the 18th century Swiss scientist Daniel Bernoulli who studied the pressures in moving fluid streams. The effect states that moving air will have a lower pressure than the still air around it. This is the principle behind how airplane wings generate lift and why beach balls stay "balanced" on top of fans in those hardware store displays.
The effect is significant in rocketry when using altimeters or any other kind of payload that senses the ambient pressure around the rocket. The air moving by the payload section can cause the payload to indicate a lower pressure than the ambient still air, thus giving a false altitude reading. The effect drops to zero at apogee when your rocket stops moving, but the altitude vs. time curve will be wrong.