In theater, an understudy, referred to in opera as cover or covering, is a performer who learns the lines and blocking or choreography of a regular actor, actress, or other performer in a play. Should the regular actor or actress be unable to appear on stage, the understudy takes over the part. Usually when the understudy takes over, the theater manager announces the cast change prior to the start of the performance. Coined in 1874, the term understudy has more recently generally been applied only to performers who can back up a role, but still regularly perform in another role.

Source: Wikipedia