The Collaborative International Dictionary of English v.0.48:

Inertia \In*er"ti*a\, n. [L., idleness, fr. iners idle. See
   Inert.]
   [1913 Webster]
   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]. The inertia of a body is proportional to its
      mass.
      [1913 Webster +PJC]

   2. Inertness; indisposition to motion, exertion, or action;
      lack of energy; sluggishness.
      [1913 Webster]

            Men . . . have immense irresolution and inertia.
                                                  --Carlyle.
      [1913 Webster]

   3. (Med.) Lack of activity; sluggishness; -- said especially
      of the uterus, when, in labor, its contractions have
      nearly or wholly ceased.
      [1913 Webster]

   Center of inertia. (Mech.) See under Center.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "inertia":
   a wise passiveness, abeyance, apathy, catalepsy, catatonia,
   cautiousness, circumspection, contemplation, contemplative life,
   creeping, deadliness, deathliness, deliberateness, deliberation,
   dilatoriness, do-nothing policy, do-nothingism, do-nothingness,
   dormancy, drawl, dullness, entropy, ergophobia, faineancy,
   faineantise, firmness, foot-dragging, hibernation, hoboism,
   idleness, immobility, immobilization, immovability, immovableness,
   inaction, inactiveness, inactivity, indifference, indolence,
   inertness, inexertion, inextricability, inflexibility,
   irremovability, just being, laggardness, laissez-aller,
   laissez-faire, laissez-faireism, languor, lassitude, latency,
   laziness, leisureliness, lentitude, lentor, listlessness,
   lotus-eating, lull, meditation, mere existence, mere tropism,
   motionlessness, neutralism, neutrality, neutralness,
   noninvolvement, nonparticipation, nonresistance, nonviolence,
   nonviolent resistance, pacifism, paralysis, passive resistance,
   passive self-annihilation, passiveness, passivism, passivity,
   pokiness, policy, procrastination, quiescence, quietism,
   reluctance, remissness, rigidity, shiftlessness, slackness, sloth,
   slothfulness, slowness, sluggardy, sluggishness, solidity,
   spring fever, stagnancy, stagnation, standpattism, stasis,
   suspense, suspension, tentativeness, torpor, underactivity,
   unmovability, unyieldingness, vagrancy, vegetation, vis inertiae,
   vita contemplativa, waiting game, watching and waiting