Plain English Meaning
An older type of pump on the engine that creates suction for some flight instruments. It uses small steel blades spinning inside a steel housing, and it needs oil to keep the metal parts from wearing each other out.
Full Definition
An older style of engine-driven air pump used to power vacuum-operated flight instruments, in which steel vanes slide in and out of slots in a rotating steel rotor to draw and push air. Because the steel vanes rub against the steel pump housing, these pumps require oil for lubrication and produce an oil-laden exhaust, which is why they are sometimes called wet-type pumps.
Context Anchor
Seen in discussions of vacuum pump systems that power such as and .