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