Opcode
The number at the start of an instruction saying which of the processor's hundreds of operations is meant.
The operation code identifies the instruction; the rest of the instruction word gives operands, addressing modes and any prefixes. The encoding space is a constant trade-off: short codes for common operations save memory and cache but make decoding irregular. Through forty years of backward compatibility, x86 has acquired an opcode map of remarkable complexity, with multi-byte prefixes and context-dependent interpretation.
The consequences are tangible. An x86 decoder needs considerably more silicon and power than a RISC-V equivalent, and because instructions vary in length you cannot tell where the next one begins without decoding the previous one. That makes disassembling x86 hard, which in turn is exploited by malware hiding instructions inside other instructions.