Source : Webster's Revised Unabridged Dictionary (1913)

Inertia \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
      inerti[ae]}.

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

            Men . . . have immense irresolution and inertia.
                                                  --Carlyle.

   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}.

Source : WordNet®

inertia
     n 1: a disposition to remain inactive or inert; "he had to
          overcome his inertia and get back to work" [syn: {inactiveness},
           {inactivity}] [ant: {activeness}]
     2: (physics) the tendency of a body to maintain is state of
        rest or uniform motion unless acted upon by an external
        force