An instruction is an order given to a computer processor by a computer program. At the lowest level, each instruction is a sequence of 0s and 1s that describes a physical operation the computer is to perform (such as "Add") and, depending on the particular instruction type, the specification of special storage areas called registers that may contain data to be used in carrying out the instruction, or the location in computer memory of data.
In a computer's assembler language, each language statement generally corresponds to a single processor instruction. In high-level languages, a language statement generally results (after program compilation) in multiple processor instructions.
In assembler language, a macro instruction is one that, during processing by the assembler program, expands to become multiple instructions (based on a previously coded macro definition).