When there is a relative velocity between a fluid and a body, the body experiences a force D, the drag force, that opposes the relative motion and points in the direction of the fluid flow.

The drag is defined by:

D=½CρAv²

C is a drag coefficient, which varies from case to case
ρ is the air density (mass per volume)
A is the effective cross-sectional area of the body
v is the velocity of the body

The drag can be used to determine things like terminal velocity.