1. Any military element whose structure is prescribed by competent authority. 2. An organization title of a subdivision of a group in a task force. 3. A standard or basic quantity into which an item of supply is divided, issued, or used. Also called unit of issue. 4. With regard to Reserve Component of the Armed Forces, a selected reserve unit organized, equipped, and trained for mobilization to serve on active duty as a unit or to augment or be augmented by another unit.