Definition
A coinbase commitment is the general technique of placing a cryptographic hash or marker inside a block's coinbase transaction so that the data inherits the block's proof-of-work without altering the block header. Because the coinbase is a leaf of the regular transaction merkle tree, anything committed there is implicitly secured by the same hashing that protects the rest of the block, and adding such data is a backward-compatible soft fork rather than a network-splitting hard fork.
Why the coinbase is the chosen home
The 80-byte block header has no spare room, and extending it would invalidate the assumptions of every node and miner. The coinbase transaction, by contrast, is uniquely flexible: it has a freely usable input script (the coinbase scriptSig) and can carry extra outputs. Committing protocol data there means older software simply ignores it as an unspendable output, while upgraded nodes enforce its meaning.
Common uses
The most widely deployed example is the SegWit witness commitment, an OP_RETURN output carrying the witness merkle root. Merged mining inserts a commitment to an auxiliary chain's block. Proposals for UTXO-set commitments and other authenticated structures also lean on the coinbase as the natural anchor point. In every case the pattern is identical: hash the data, place the hash in the coinbase, and let proof-of-work do the rest.
For the canonical instance, see the witness commitment, and for a forward-looking application see the UTXO commitment.
In Simple Terms
A coinbase commitment is the general technique of placing a cryptographic hash or marker inside a block’s coinbase transaction so that the data inherits the…