A gerotor is a positive displacement pump. The name gerotor is derived from "generated rotor". A gerotor unit consists of an inner and outer rotor. The inner rotor has n teeth, while the outer rotor has n+1 teeth; with n defined as a natural number greater than or equal to 2. The axis of the inner rotor is offset from the axis of the outer rotor and both rotors rotate on their respective axes. The geometry of the two rotors partitions the volume between them into n different dynamically-changing volumes. During the assembly's rotation cycle, each of these volumes changes continuously, so any given volume first increases, and then decreases. An increase creates a vacuum. This vacuum creates suction, and hence, this part of the cycle is where the inlet is located. As a volume decreases compression occurs. During this compression period, fluids can be pumped, or, if they are gaseous fluids, compressed.
A gerotor can also function as a pistonless rotary engine. High pressure gas enters the intake and pushes against the inner and outer rotors, causing both to rotate as the volume between the inner and outer rotor increases. During the compression period, the exhaust is pumped out.
At the most basic level, a Ge-rotor is essentially one that is moved via fluid power. Originally this fluid was water, today the wider use is in hydraulic devices. Myron F. Hill, who might be called the father of the Ge-rotor, in his booklet "Kinematics of Ge-rotors," lists efforts by Galloway in 1787, by Nash and Tilden in 1879, by Cooley in 1900, by Professor Lilly of Dublin University in 1915, and by Feuerheerd in 1918. These men were all working to perfect an internal gear mechanism by a one-tooth difference to provide displacement.
Myron Hill made his first efforts in 1906, then, in 1921, gave his entire time to developing the Ge-rotor. He developed a great deal of geometric theory bearing upon these rotors, coined the word GE-ROTOR (meaning "Generated Rotor") and secured basic patents on GE-ROTOR.