The Collaborative International Dictionary of English v.0.48:

Dismount \Dis*mount"\, v. i. [imp. & p. p. Dismounted; p. pr.
   & vb. n. Dismounting.] [Pref. dis- + mount: cf. OF.
   desmonter, F. d['e]monter.]
   1. To come down; to descend. [Poetic]
      [1913 Webster]

            But now the bright sun ginneth to dismount.
                                                  --Spenser.
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   2. To alight from a horse; to descend or get off, as a rider
      from his beast; as, the troops dismounted.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Dismount \Dis*mount"\, v. t.
   1. To throw or bring down from an elevation, place of honor
      and authority, or the like.
      [1913 Webster]

            Dismounted from his authority.        --Barrow.
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   2. To throw or remove from a horse; to unhorse; as, the
      soldier dismounted his adversary.
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   3. (Mech.) To take down, or apart, as a machine.
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   4. To throw or remove from the carriage, or from that on
      which a thing is mounted; to break the carriage or wheels
      of, and render useless; to deprive of equipments or
      mountings; -- said esp. of artillery.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dismount
    n 1: the act of dismounting (a horse or bike etc.)
    v 1: alight from (a horse) [syn: unhorse, dismount, light,
         get off, get down]