In physics, motion is a change in position of an object with respect to time and its reference point. Motion is typically described in terms of displacement, velocity, acceleration, and time. Motion is observed by attaching a frame of reference to a body and measuring its change in position relative to another reference frame.

A body which does not move is said to be at rest, motionless, immobile, stationary, or to have constant (time-invariant) position. An object's motion cannot change unless it is acted upon by a force, as described by Newton's first law. An object's momentum is directly related to the object's mass and velocity, and the total momentum of all objects in a closed system (one not affected by external forces) does not change with time, as described by the law of conservation of momentum.

As there is no absolute frame of reference, absolute motion cannot be determined. Thus, everything in the universe can be considered to be moving.

More generally, the term motion signifies a continuous change in the configuration of a physical system. For example, one can talk about motion of a wave or a quantum particle (or any other field) where the configuration consists of probabilities of occupying specific positions.