Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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