Solenoid valves (sometimes called simply "solenoids") are electromechanically controlled fluid regulators. Used in a wide range of devices from motors to washing machines to sprinklers, they come in many types—varying in their size, complexity, energy output, and compatibility with different kinds of fluid.
Their central jobs are to block, allow, or mix the liquids in part of a machine or system. Some use a plunger like linear mechanical motion, others pivot or rock. Some run on AC, some DC, and the strength of their magnetic field varies by application. Some are two way, some three or more, and they're typically either referred to as normally open (N.O.) or normally closed (N.C.).
First built in 1910 by ASCO Numatics, these valves have since been broadly adopted for their reliability, efficiency, and long service life, and today are widely integrated into fluid-based mechanical systems on all scales, from commercial to industrial.