From David Bolton,
Your Guide to C / C++ / C#.
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Definition: A register is part of a CPU and holds a binary value. The size of a register is a multiple of 8 bits. Older CPUs had 8 or 16 bit registers, but 32 bits or 64 bits are now common.

Operations in a register are much faster than on a location in RAM. CPUs have multiple registers to optimize processing. Much of an application's code pulls values into one or more registers from RAM, does some processing on those registers and then writes the value back.

Glossary:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Also Known As: Accumulator

Examples: The EAX register is a general purpose register in Intel and AMD processors.