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An instruction is, at the highest level of abstraction, the smallest unit of computation which a programmer can direct the CPU to perform.

In more practical terms, the CPU operates by fetching items from main memory; each item contains a single directive for the CPU (e.g. a the contents of one register to another, or move a unit of data from a register, to main memory) - those items are instructions. (An instruction is usually a word, but on some CPUs, multiple instructions could be packed in each word, while on others, some instructions took multiple words.)