1. A piece of cloth, rope, or nylon used to mark a position on a wireline. 2. Strips of plastic, cloth, or paper used to mark instrument or source locations or to indicate access points in the field. 3. A bit attached to a computer word to indicate the boundary of a field. 4. An indicator to tell a later part of a computer program about a condition that occurred earlier in the program. Used to synchronize within a program or to lock out data. Also called semaphore.