Stands for “Central Processing Unit.” This is the pretty much the brain of your computer. It processes everything from basic instructions to complex functions. Any time something needs to be computed, it gets sent to the CPU. Every day, it’s compute this, compute that — you’d think the CPU would need a break after awhile. But no — it just keeps on processing. The CPU can also be referred to simply as the “processor.”
Source: http://www.techterms.com/definition/cpu