A valve is an apparatus or natural object that opens, closes, or partially blocks different passages to regulate, direct, or control the flow of a fluid (gases, liquids, fluidized solids, or slurries). Although they are considered a distinct category, valves are really fittings. Fluid moves from greater pressure to lower pressure through an open valve. The name comes from the Latin valva, which means a door’s moving portion, and volvere, which means to turn or roll.