See standby.

(hot stand´bī) (n.) A method of redundancy in which the primary and secondary (i.e., backup) systems run simultaneously. The data is mirrored to the secondary serverin real time so that both systems c [..]

Backup equipment kept turned on and running in case some equipment fails Hot Swappable

  A contingency/fallback approach to maintaining system availability whereby a second system, with the same configuration as the main system is kept running - often 'mirroring' the processing of the main system - ready to take over the processing load instantaneously, should there be any failure in the main system. *** The Information Sec [..]

Hot standby is a redundant method in which one system runs simultaneously with an identical primary system. Upon failure of the primary system, the hot standby system immediately takes over, replacing [..]