In*er"ti*a (?), n. [L., idleness, fr. iners idle. See Inert.]

1. (Physics) That property of matter by which it tends when at rest to remain so, and when in motion to continue in motion, and in the same straight line or direction, unless acted on by some external force; -- sometimes called vis inertiƦ.

2. Inertness; indisposition to motion, exertion, or action; want of energy; sluggishness.

Men . . . have immense irresolution and inertia.

3. (Med.) Want of activity; sluggishness; -- said especially of the uterus, when, in labor, its contractions have nearly or wholly ceased.

Center of inertia. (Mech.) See under Center.