channel or pipe for conveying water or other fluid or for carrying out certain other purposes, such as protecting electric cables.

In water-supply systems the term is usually reserved for covered or closed sections of aqueduct, especially those that transport water under pressure. Large conduits may be fabricated of steel sections joined in the field or of reinforced concrete, often precast and prestressed, or they may be driven at depth through solid rock, for example, under mountainous terrain or under rivers. Smaller pressure conduits are usually made of cast iron, steel, or asbestos cement. Pressure conduits are usually kept below the hydraulic grade line—that is, the line representing the height to which the water would rise if free—to avoid possible entrapment of air.