GlossaryAdvanced
OP_RETURN
A Bitcoin Script opcode that lets a small amount of arbitrary data be embedded in a transaction, which is provably unspendable.
OP_RETURN is a Bitcoin Script opcode (see Kaavon's "Bitcoin Script Explained" guide) that lets a transaction embed a small amount of arbitrary data, while marking the output containing it as provably unspendable — explicitly signaling to the network that no funds are actually associated with that specific output.
Because the data is provably unspendable, OP_RETURN outputs don't bloat Bitcoin's set of spendable transaction outputs (the UTXO set) the way an ordinary, technically-spendable output misused for data storage would.
Examples
- OP_RETURN has historically been used to timestamp documents or embed small references to off-chain data directly in a Bitcoin transaction.
- An OP_RETURN output's explicit unspendability is what prevents this kind of data embedding from bloating Bitcoin's UTXO set the way a misused ordinary output would.