An item of solid one-piece construction, designed to restrict the flow of a fluid through it. it has an orifice of a smaller size than the diameter of flow into it. excludes items such as valve (1), (as modified); and others with an adjustable orifice. excludes pipe, tube, or hose fittings with restricted passages and items designed to test fluid flow. for items containing screens or filters, see restrictor unit, fluid flow.