Definition
The coinbase transaction (not to be confused with the exchange Coinbase) is a special transaction that appears as the first entry in every Bitcoin block. Unlike regular transactions, it has no inputs from previous transactions. Instead, it creates new Bitcoin out of thin air as the block subsidy, plus collects all transaction fees from the block.
Miners can include arbitrary data in the coinbase transaction’s input field. Satoshi Nakamoto famously included a newspaper headline in the genesis block’s coinbase: a reference to bank bailouts. The extra nonce field in the coinbase is also used to expand the search space beyond the 32-bit header nonce.
In Simple Terms
The special first transaction in each block that creates new Bitcoin and pays the miner.
The first transaction in every block, created by the miner. It has no inputs and creates new Bitcoin as the block reward, paid to the miner's address.