UNDERSTUDY

An 'understudy' is a theatrical term for someone who learns the lines and blocking/choreography of a leading actor or actress in a theatrical play. Should the lead actor or actress be unable to appear on stage because of illness or accident, the understudy takes over the part. Usually, when the understudy takes over, the theater manager will make the announcement prior to the start of the performance. Several actors made their name in show-business by being the understudy of a leading actor and taking the role over for several performances (for example, Anthony Hopkins with Laurence Olivier).
More recently, the term "understudy" has generally only been applied to performers who will cover a part, but still regularly perform in another role within the show; usually a minor, extra role. Performers who are only committed to covering a part and do not regularly appear in the show are often referred to as 'standbys'. Standbys are normally required to sign-in and remain at the theater the same as other cast members, although sometimes they may call in, until they are released by the Production Stage Manager. If there is no doubt about the health of the actor being "covered," or there are no hazardous stunts to be performed, a standby may be released at the first intermission if not before.
In musical theatre, the term 'swing' is often used to refer to a member of the company who understudies several chorus and/or dancing roles.
In the opera world, the term used is 'cover' or 'covering'.