The Collaborative International Dictionary of English v.0.48:

Stowaway \Stow"a*way`\, n.
   One who conceals himself board of a vessel about to leave
   port, or on a railway train, in order to obtain a free
   passage.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stowaway
    n 1: a person who hides aboard a ship or plane in the hope of
         getting free passage