air resistance
Air resistance is the pushback you feel from the air as you move through it: the wind in your face on a bike, the tug on your hand held out a car window. It is a force that opposes an object's motion through the air, and it grows stronger the faster you go.
Air resistance (a form of drag) is a force directed opposite to an object's velocity, caused by the object having to push air out of the way. At the everyday speeds of falling balls and moving cars, its magnitude grows roughly with the square of the speed, F_drag proportional to v^2, and it also depends on the object's cross-sectional area, its shape, and the air's density. Larger, blunter, faster objects feel more drag.
Air resistance is why a feather flutters down while a coin drops fast: the feather's large area and tiny weight make drag matter enormously. In introductory problems we often say 'neglecting air resistance' to keep things simple, but in real life it is what sets a skydiver's terminal velocity and shapes cars, planes, and bullets.
Drop a flat sheet of paper and a crumpled ball of the same paper: the ball falls much faster. Same weight, but the flat sheet meets far more air resistance.
Air resistance depends on shape and area, not just weight.
Air resistance is a nonconservative force that removes mechanical energy as heat and sound. The tidy 'v^2' rule is an approximation valid for ordinary sizes and speeds; at very low speeds or tiny scales, drag can instead grow in proportion to v.