In computer engineering, an execution unit is a part of a CPU (not to be confused with the CPUs main control unit) that performs specific operations or calculations. It may have its own internal control sequence unit, some registers, and other internal units such as a sub-ALU or FPU, or some smaller, more specific components.
Now modern CPUs have multiple parallel execution units, referred to as scalar or superscalar design. The simplest arrangement is to use one, the bus manager, to manage the memory interface, and the others to perform calculations. Additionally, modern CPUs execution units are usually pipelined.