Gerotor motor: a gear-within-gear pump that consists of an inner gear with one less tooth than the outer gear. A positive displacement pump.
The name gerotor comes from the term, "Generated Rotor". A gerotor unit consists of an inner and outer rotor; the inner rotor has one less tooth than the outer rotor. The axis of the inner rotor is offset from the axis of the outer rotor and both rotate on their respective axes. The geometry of the two rotors partitions the volume between them into different dynamically-changing volumes.
These volumes change continuously during the rotation cycle; so any given volume first increases, and then decreases. An increase creates a vacuum; this vacuum creates suction, and this part of the cycle is where the intake is located. As a volume decreases, compression occurs. During this compression period, fluids can be pumped, or, if they are gaseous fluids, compressed.