Opcode

An opcode is a single instruction in Bitcoin Script, the stack-based language that defines the conditions for spending coins. Each opcode occupies one byte, giving a possible space of 256 operations; Satoshi Nakamoto disabled about 15 of them in 2010 after security review, and roughly 100 remain in active use.

Why it matters

Opcodes are the vocabulary of bitcoin ownership. Common operations verify signatures, hash data, enforce timelocks and evaluate branching conditions, and every wallet balance is really an output guarded by a small opcode program. Script is deliberately limited: it has no loops and is not Turing complete, so every script's execution cost is bounded and predictable, closing the door on the unbounded-computation vulnerabilities that have plagued more expressive systems.

New opcodes arrive rarely and by soft fork, as with the timelock opcodes added in 2015 and 2016 and Taproot's script upgrades in 2021. Current proposals such as OP_CTV and OP_CAT would extend Script toward covenants.

In the gold vs bitcoin debate

Opcodes give bitcoin a property gold lacks entirely: ownership conditions that execute themselves. Multisignature treasuries, inheritance timelocks and payment channels are all opcode constructions. Gold's simplicity means it cannot malfunction, which its advocates value; bitcoin wagers that a small, auditable instruction set captures most of programmability's benefit with little of its risk.