in.er.tia: n. 1. Phys. The tendency of a body at rest to remain at rest or of a body in motion to stay in motion in a straight line unless acted on by an outside force. 2. Resistance or disinclination to motion, action, or change: bureaucratic inertia. [Lat., idleness < iners, inert-, inert. See INERT.] -- in.er'tial adj. -- in.er'tial.ly adv.