The force that acts on an object moving through a fluid, opposite to its motion, is drag.
Drag is a resistive force produced when an object moves through air, water, or another fluid. It depends on the object's speed, shape, size, and the fluid's density and viscosity. A swimmer feels water drag, while a falling skydiver experiences air drag.
At relatively low speeds, drag can be approximately proportional to speed; at higher Reynolds numbers, it commonly follows a square-of-speed relationship. This is why doubling a car's speed can require much more power to overcome air resistance. The standard drag equation includes fluid density, speed, cross-sectional area, and a dimensionless drag coefficient.
Drag is often confused with lift. Lift acts perpendicular to fluid flow, whereas drag acts parallel to the flow and opposes relative motion. Buoyancy is different again: it is an upward force caused by pressure differences in a fluid, not the general resistive force described here.