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 or actress in a play. Should the regular actor or actress be unable to appear on stage because of illness, emergencies or death, the understudy takes over the part. Usually when the understudy takes over, the theater manager will make a relevant announcement prior to the start of the performance. First 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, usually a minor one.
Performers who are only committed to covering a part and do not regularly appear in the show are often referred to as standbys and alternates . 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. At times, standbys are required to stay within a certain area around the theater (10 blocks in New York City is a common standard). Today, the standbys must also have a cell phone so that at any time they can be called to the theater.
Alternates, like standbys, do not have a regular character in the production, but they are scheduled to go on for a physically and vocally challenging role for a certain number of performances a week. Examples of this are the title role in Evita, Christine in The Phantom of the Opera (1986 musical), and Peter Parker/Spider-Man in Spider-Man: Turn Off the Dark.
In musical theater, the term swing is often used for a member of the company who understudies several chorus and/or dancing roles.  If an understudy fills in for a lead role, a swing will act the parts normally performed by the understudy. A super swing or universal swing is a swing who may commute around the country as needed to act in various productions of a widespread show.
In contrast, a prompt cues an actor while not personally being on the stage or in the spotlight.
In some instances, a lead role will be covered by multiple understudies. The second (sometimes third, or even fourth) understudy will only perform if both the principal actor and the first (or second, or third) understudy are unable to perform.
Several actors made their name in show business by being the understudy of a leading actor and taking the role over for several performances, including: Anthony Hopkins for Laurence Olivier, when Olivier became ill with cancer during the run of the National Theater's The Dance of Death , 1967; Ted Neeley for Jeff Fenholt during the 1971 Broadway run of Jesus Christ Superstar when Neeley was asked to star in the 1973 film version and subsequent tours; and Edward Bennett for David Tennant as Hamlet in the RSC's 2008 production. Kerry Ellis was called to perform as Eliza Doolittle in My Fair Lady when Martine McCutcheon took ill. In the audience that day was Brian May, who was then writing his musical We Will Rock You , and he was so impressed with Ellis' performance he immediately wanted to cast her as Meat, a lead in the show.
When Carol Haney broke her ankle while playing the role of Gladys in The Pajama Game , Shirley MacLaine assumed the role. In addition, Arthur Jefferson aka Stan Laurel was an understudy of Charles Chaplin working for Fred Karno, the music hall impresario before they entered American film.
Merwin Foard, a Broadway veteran, was a standby and understudy for thirty roles. He was featured in Stephanie Rigg's "The Standbys."