in·er·tia / iˈnərshə/ • n. 1. a tendency to do nothing or to remain unchanged: the bureaucratic inertia of government.2. Physics a property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force.See also moment of inertia. ∎  resistance to change in some other physical property: the thermal inertia of the oceans will delay the full rise in temperature for a few decades.DERIVATIVES: in·er·tia·less adj.

inertia Property possessed by all matter that is a measure of the way an object resists changes to its state of motion. Isaac Newton formulated the first law of motion, sometimes called the law of inertia, stating that a body will remain at rest or in uniform motion unless acted upon by external forces.

inertia (in-er-shă) n. (in physiology) sluggishness or absence of activity in certain smooth muscles. uterine i. inertia of the muscular wall of the uterus during labour, making the process excessively long. It may be present from the start of labour or it may develop because of exhaustion following strong contractions.