In Computer architecture, an instruction is a single operation of a processor. The width of an instruction depends on the architecture of the platform, but it is usually from 4 to 64 bits wide. The content of an instruction is determined by the platform's Instruction Set Architecture (ISA), but it usually contains an opcode, which determines the type of instruction, register sources and destination operands, and maybe an immediate field.

At a higher level, in computer science, and instruction could also be part of a computer program in a programming language; See also Procedure.