A duplicate of important information, such as a computer program or data, is a backup.
A duplicate copy of a program, a disk, or data, made either for archiving purposes or for safeguarding valuable files from loss should the active copy be damaged or destroyed. A backup is an “insurance” copy. Some application programs automatically make backup copies of data files, maintaining both the current version and the preceding version on disk. Also called: backup copy, backup file.
(Computer science) A copy of a file or directory on a separate storage device; SYN. computer backup.
A system, device, file or facility that can be used as an alternative in case of a malfunction or loss of data.
In computing, a copy file that is transferred to another medium, usually a floppy disc or tape. The purpose of this is to have available a copy of a file that can be restored in case of a fault in the system or the file itself. Backup files are also created by many applications (with the extension .BAC or .BAK); a version is therefore available of the original file before it was modified by the current application. SYN. back-up, back up.

1. Someone or something that supports or is ready to provide support in a conflict or a confrontation by doing some work or a difficult assignment is a backup.
2. When a piece of equipment or machinery is very important and another is kept available in case the one that is normally used fails, the second piece of equipment or machinery is a backup.
3. A backup is a situation in which something is being prevented from moving, progressing, or flowing normally.