Definitions of backup:

• noun:   (computer science) a copy of a file or directory on a separate storage device

  Example: "He made a backup in case the original was accidentally damaged or erased"

• noun:   an accumulation caused by clogging or a stoppage

  Example: "A traffic backup on the main street"

• noun:   the act of providing approval and support
• noun:   a subordinate musical part; provides background for more important parts
• noun:   someone who takes the place of another (as when things get dangerous or difficult)