Inertia, n.
Etymology: [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 inertiæ.
2. Inertness; indisposition to motion, exertion, or action; want of energy; sluggishness. Men . . . have immense irresolution and inertia. Carlyle.
3. (Med.)
1 more senses
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.
Shades of meaning
Fernald lists inertia only as an antonym, under act.
He lists it without comment; the entries above explain its neighbors.