Pertaining to the operation of a functional unit when under the direct control of the system with which it is associated. Note 1: On-line units are available for immediate use on demand by the system without human intervention. Note 2: On-line units may not be independently operated. 2. Pertaining to equipment that is connected to a system, and is in operation. 3. In computer technology, the state or condition of a device or equipment that is under the direct control of another device. 4. In computer technology, the status of a device that is functional and ready for service. Also spelled "on line" and "online. "