backup (n.)Related entries & more
"a standby, a reserve," 1952; see back up (v.). Specific reference to computing is from 1965.
stand-by (n.)Related entries & more
also standby, 1796, "that which stands by one," originally nautical, of a vessel kept nearby for emergencies, from verbal phrase stand by "await, support, remain beside" (mid-13c.); see stand (v.) + by. Meaning "state of being ready for duty" is from 1946. In civil aviation, as an adjective meaning "without a booked ticket," from 1961. As an order to hold one's self in readiness, it is recorded from 1660s.