Inertia

An object's resistance to a change in its velocity. The tendency of an object at rest to remain at rest, and of an object in motion to remain in motion, in the absence of applied forces. Newton’s first law of motion stated this tendency, while his second law effectively states that momentum (the quantification of inertia) is proportional to both the mass and the velocity of an object.

See also: Newtons Laws of Motion, Rotational Inertia.