Backup singer, backing singer, background singer, and background vocalist are all terms meaning the same: a vocalist who sings in support (in back-up) of a lead vocalist or singer, often accompanied by other backup singers.
In music a singer or vocalist is a type of musician who uses his or her voice as an instrument to make music.
A lead singer (in barbershop music simply called a lead) is one who sings the primary vocals of a song, as opposed to a backupsinger who sings backup vocal(s) to a song or harmonies to the lead singer.
Singers may also be classified according to the style of music they sing, such as soul singers or carnatic vocalists.
Backup in computer engineering refers to the copying of data for the purpose of having a second copy of an original source, in case of damage to the original data source.
A backup vocalist is a vocalist who sings in harmony with the lead vocalist, other backup vocalists, or alone but in the background of a song.
Backupsinger, backing singer, background singer, and background vocalist are all terms with the same meaning: a vocalist who sings in support (in back-up) of a lead vocalist or singer, often accompanied by other backupsingers.