instruction prefix
An extra byte in front of an x86-64 instruction that changes it, for example making it work on 16 bits instead of 32.
An x86 instruction prefix is a byte before the opcode that modifies the instruction: 66h selects 16-bit operand size, F0h is lock, F2h/F3h are rep prefixes or select SSE forms, and 2Eh-65h are segment overrides.
See also