Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

understudy
     n : an actor able to replace a regular performer when required
         [syn: {standby}]
     v : be an understudy or alternate for a role [syn: {alternate}]
     [also: {understudied}]