Definition of three-way valve

1. a type of shutoff valve having two extreme positions, each of which either (1) directs an incoming single-gas stream to one of two alternate outlet ports, or (2) directs two different incoming gas streams alternately to a single outlet port. Some designs of three-way valves incorporate a position in which all ports are closed to flow. [SEMI Chemicals/Gases, Vol. 1, 1990 (no longer in print)]