1. Portion of a body of troops that is kept to the rear, or withheld from action at the beginning of an engagement, in order to be available for a decisive movement. 2. Members of the uniformed Services who are not in active service but who are subject to call to active duty. 3. Portion of an appropriation or contract authorization held or set aside for future operations or contingencies and, in respect to which, administrative authorization to incur commitments or obligations has been withheld.