Real Dictionary

understudy
countable noun: An actor's or actress's understudy is the person who has learned their part in a play and can act the part if the actor or actress is ill.

understudy in American English
an actor who learns the part of another actor so as to serve as a substitute when necessary
to act as an understudy (to)
intransitive verb: to act or work as an understudy
noun: a performer who learns the role of another in order to serve as a replacement if necessary
transitive verb: to learn (a role) in order to replace the regular actor or actress when necessary

understudy in British English
noun: an actor or actress who studies a part so as to be able to replace the usual actor or actress if necessary
verb: to study (a role or part) so as to be able to replace the usual actor or actress if necessary