Armed forces that are not on active duty but can be called in an emergency WordNet 3.0 & Open English WordNet

A military reserve, active reserve, reserve formation, or simply reserve, is a group of military personnel or units that is initially not committed to a battle by its commander, so that it remains available to address unforeseen situations or exploit sudden opportunities. Wikipedia

Military units not initially committed to battle Wikipedia Disambiguation

A body of troops kept in the rear of an army drawn up for battle, reserved to support the other lines as occasion may require; a force or body of troops kept for an exigency. Wiktionary

Military: body of troops in the rear of an army. Wiktionary (translation)