1. In cryptography, an entity whose public key is certified in a public key certificate. 2. Generally, a person, process, or device, causing information to flow among objects or change to the system state. 3. An active entity, generally in the form of a person, process, executing program or device that causes information to flow among objects or changes the system state. Examples: process, executing program. See also: Object. 4. Abbreviation of security subject. 5. An active entity, generally in the form of a person, process, or device. 6. An active entity that causes information to flow between objects or changes the system state; a process. 7. An active entity, generally in the form of a person, process, or device that causes information to flow among objects or changes the system state. Technically, a process/domain pair.