gen. (That may be) held in reserve; constituting a reserve (sense A. 2b); spare.

Of a thing (esp. a vehicle, vessel, piece of equipment or technology, etc.): ready to be deployed immediately, esp. as a replacement or reserve in…

Originally U.S. A stand-by, reserve. Also attributive. spec. in Computing, (the making of) a duplicate copy of a disc, file, etc., for use in case…