a: a property of matter by which it remains at rest or in uniform motion in the same straight line unless acted upon by some external force b: an analogous property of other physical quantities (as electricity)
: lack of activity or movement—used especially of the uterus in labor when its contractions are weak or irregular
Inherent property of a body that makes it oppose any force that would cause a change in its motion. A body at rest and a body in motion both oppose forces that might cause acceleration. The inertia of a body can be measured by its mass, which governs its resistance to the action of a force, or by its moment of inertia about a specified axis, which measures its resistance to the action of a torque about the same axis.