Crypto glossary
Coinbase transaction
A coinbase transaction is the first transaction in every Bitcoin block, which creates the block subsidy and pays it, plus all fees, to the miner.
Unlike normal transactions, a coinbase transaction has no real inputs: it creates new coins. The miner who builds the block writes it to pay the block subsidy plus the fees of every transaction in the block to addresses of its choosing, usually controlled by its mining pool. If the claimed amount exceeds what the rules allow, the whole block is invalid.
The coinbase input contains a free-form field that miners use for the block height, required since BIP-34, extra nonce space and short messages. Satoshi Nakamoto embedded a newspaper headline about bank bailouts in the genesis block's coinbase. Pools still tag their blocks here, which is how explorers attribute blocks to them, and on SegWit blocks the coinbase also commits to the witness data.
Coins from a coinbase transaction cannot be spent until 100 more blocks have been mined, so that rewards from blocks that become stale vanish without breaking later transactions. The term has nothing to do with the exchange of the same name, which took its name from this concept.
Example
In block 840,000, the halving block, the coinbase transaction paid the miner 3.125 BTC of new subsidy plus a record amount of fees generated by the launch of Runes.