The Collaborative International Dictionary of English v.0.48:

Override \O`ver*ride"\, v. t. [imp. Overrode; p. p.
   Overridden, Overrode, Overrid; p. pr. & vb. n.
   Overriding.] [AS. offer[imac]dan.]
   1. To ride over or across; to ride upon; to trample down.
      [1913 Webster]

            The carter overridden with [i. e., by] his cart.
                                                  --Chaucer.
      [1913 Webster]

   2. To suppress; to destroy; to supersede; to annul; to
      nullify; as, one law overrides another; to override a
      veto.
      [1913 Webster]

   3. Hence: To countermand; to overrule; as, a supervisor may
      override the decision of a subordinate.
      [PJC]

   4. To replace (one system with another); as, the pilot
      overrode the automatic pilot and took manual control of
      the airplane.
      [PJC]

   5. To ride beyond; to pass; to outride. [Obs.]
      [1913 Webster]

            I overrode him on the way.            --Shak.
      [1913 Webster]

   6. To ride too much; to ride, as a horse, beyond its
      strength.
      [1913 Webster]

WordNet (r) 3.0 (2006):

override
    n 1: a manually operated device to correct the operation of an
         automatic device
    2: the act of nullifying; making null and void; counteracting or
       overriding the effect or force of something [syn:
       nullification, override]
    v 1: rule against; "The Republicans were overruled when the
         House voted on the bill" [syn: overrule, overturn,
         override, overthrow, reverse]
    2: prevail over; "health considerations override financial
       concerns"
    3: counteract the normal operation of (an automatic gear shift
       in a vehicle)
    4: ride (a horse) too hard

Moby Thesaurus II by Grady Ward, 1.0:

138 Moby Thesaurus words for "override":
   abolish, abrogate, annul, arch over, beat down, bend, bestraddle,
   bestride, break, bridge, browbeat, bulldoze, bully, cancel,
   castrate, clamp down on, coerce, compel, conquer, countermand,
   counterorder, cow, crush, daunt, despotize, disannul, do away with,
   domineer, domineer over, enslave, extend over, fell, flatten,
   grind, grind down, hang over, henpeck, hit and run, humble,
   imbricate, intimidate, inundate, invalidate, jut, keep down,
   keep under, lap, lap over, lie over, lord it over, make void,
   master, nullify, oppress, outclass, outdo, outgo, outjump, outleap,
   outmaneuver, outmarch, outpace, outperform, outplay, outpoint,
   outrange, outreach, outride, outrival, outrun, outshine, outstep,
   outstrip, outvie, outwit, overarch, overawe, overbear, overflow,
   overhang, overjump, overlap, overleap, overlie, overmaster,
   overmatch, overplay, overrule, overrun, press heavy on, prostrate,
   put down, quell, recall, recant, reduce, renege, repeal, repress,
   rescind, retract, reverse, revoke, ride, ride down, ride over,
   ride roughshod over, run down, run over, set aside, shingle, smash,
   span, subdue, subjugate, suppress, suspend, terrorize, trample,
   trample down, trample on, trample underfoot, trample upon,
   tread down, tread underfoot, tread upon, tyrannize, tyrannize over,
   unman, vacate, vanquish, void, waive, walk all over, walk over,
   weigh heavy on, withdraw, write off