Central Processing Unit
From Unofficial BOINC Wiki
That part of the computer system that performs all of the main operations required by the software running on the computer. This term is not restricted in its use to only the main computing unit of a computer. This term can also be used to designate the main processing unit of a smaller component of the computer as a whole. So we have the Central Processing Unit of the computer as a whole, and we can also have a Central Processing Unit within a subsystem such as the video controller.
The Central Processing Unit (CPU) is the "brain" of the computer and is the component that performs the work done by the machine in response to the commands in a program.
The speed of the CPU is measured in clock frequency, normally expressed in gigahertz (GHz) or in MIPS, or Millions of Instructions per Second. The speed of older CPUs may be measured in megahertz (MHz) or even kilohertz (KHz). Especially today, a higher CPU speed does not necessarily mean faster completion of a workunit. Other factors such as CPU design, motherboard and system RAM also play a part in 'crunching speed'.