The annulus area in a compressor refers to the space between the rotor and the casing, essentially the flow area within the compressor. This term is specific to gas turbine engines, distinguishing it from other types like reciprocating compressors. The annulus is defined by two concentric circles formed by the rotor and the compressor case. Understanding the annulus area is crucial for analyzing compressor performance, particularly in relation to factors such as annulus-area ratio and blade design. Accurate calculations of this area are essential for optimizing gas turbine engine efficiency.