The Collaborative International Dictionary of English v.0.48:

Deserter \De*sert"er\ (d[-e]*z[~e]rt"[~e]r), n.
   One who forsakes a duty, a cause or a party, a friend, or any
   one to whom he owes service; especially, a soldier or a
   seaman who abandons the service without leave; one guilty of
   desertion.
   [1913 Webster]

WordNet (r) 3.0 (2006):

deserter
    n 1: a disloyal person who betrays or deserts his cause or
         religion or political party or friend etc. [syn:
         deserter, apostate, renegade, turncoat, recreant,
         ratter]
    2: a person who abandons their duty (as on a military post)
       [syn: deserter, defector]