The Collaborative International Dictionary of English v.0.48:

Understudy \Un"der*stud`y\, v. t. & i. (Theater)
   To study, as another actor's part, in order to be his
   substitute in an emergency; to study another actor's part.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Understudy \Un"der*stud`y\, n.
   One who studies another's part with a view to assuming it in
   an emergency.
   [1913 Webster]

WordNet (r) 3.0 (2006):

understudy
    n 1: an actor able to replace a regular performer when required
         [syn: understudy, standby]
    v 1: be an understudy or alternate for a role [syn:
         understudy, alternate]