Hence second string, a second resource available if the first should fail. Frequently (with hyphen) attributive.

A member of a garrison, a guard. Also figurative.

Something that can be relied on; a main support; a chief resource. Frequently with modifying adjective.

Something upon which one may fall back, or to which one may resort; an alternative plan or course of action that may be used in an emergency or…

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…