Synonyms for Backup:

adj.

substitute (adjective)
alternate, substitute.
supplementary (adjective)
auxiliary.

n.

more, too, overmuch, excessive, Hyper-, exceptional, beyond, surplus, over, overly. cookie, supplementary, emergency, configuration file, access profile, bitmap, batch file, secondary, archive, bitstream, ASCII file, increase, standby, supplemental, checksum. blues, acid jazz, acid house, bebop, background music, bhangra, bluegrass, ambient. agent (noun)
proxy.
assistant (noun)
lieutenant, helper.
auxiliary (noun)
substitute, alternate.
backup (noun)
relief, stand in, fill in, backup man, computer backup, substitute, reliever.
deputy (noun)
deputy, spokesman, dummy, emissary, alternate, token, aide, helpmate, double, spokeswoman, nominee, stand in, agent, servant, envoy, acolyte, assignee, second, surrogate, assistant, substitute, attorney, attendant, delegate, representative, proxy, arm, helper, lieutenant, ghost, understudy, mouthpiece, figurehead, appointee, auxiliary.
help (noun)
contribution, help, aid, assistance, service, a good/bad turn, buddy system, cooperation, hand.
substitute (noun)
understudy, standby.

Other synonyms:

cookie, checksum, archive, standby, bluegrass, batch file, background music, ASCII file, bitstream, acid jazz, configuration file, acid house, bebop, bitmap, ambient, bhangra, access profile. blues. secondary. Other relevant words:
checksum, aid, supplemental, blues, supplementary, standby, cooperation, bebop, ambient, backup man, reliever, bluegrass, cookie, help, bitstream, archive, relief, assistance, secondary, computer backup, bhangra, emergency.

Usage examples for backup

  1. More often, missionaries end up getting refreshed from a backup after they aren't heard from for a decade or so. – Down and Out in the Magic Kingdom by Cory Doctorow