This chapter and the following three chapters deal with the rotating modules of both aeroengines and gas turbines. These four chapters discuss compressors, fans, and turbines. Since all of these are identified as turbomachines, it is necessary here to introduce a definition for turbomachines. The word turbo (or turbines) is of Latin origin meaning “that which spins or whirls around.” Turbomachines are often referred to as rotor dynamic devices that transfer energy to or from a working fluid through the forces generated by a rotor. For compressors or fans, energy is transferred from the rotor to the fluid. The reverse occurs in turbines, which deliver shaft power in exchange of thermal energy taken from the working fluid.