What term is used for the force that opposes the motion of an object?

The term that describes the force opposing the motion of an object is friction. Friction occurs when two surfaces come into contact and resist sliding past each other. It can be affected by the nature of the surfaces and the presence of any lubricants. This resistive force acts in the opposite direction of the object's motion, thereby slowing it down or preventing it from moving.

In contrast to friction, acceleration refers to the rate of change of velocity of an object, indicating how quickly it speeds up or slows down, but does not describe a resisting force. Gravity is the force of attraction between two masses, such as the Earth and an object, directing them toward each other but not necessarily opposing motion in the context of a surface interaction. Inertia is the tendency of an object to resist changes in its motion, meaning it wants to keep moving at a constant velocity unless acted upon by an external force, but it is not a force itself. Thus, friction is the clearest answer for a force acting against motion.