Child Pays For Parent (CPFP)

Child Pays For Parent (CPFP) is a fee bumping technique where a user spends an output from a low-feerate unconfirmed transaction in a child transaction with a high feerate in order to encourage miners to include both transactions in a block.

Bitcoin consensus rules require that the transaction which creates an output must appear earlier in the block chain than the transaction which spends that outputs—including having the parent transaction appear earlier in the same block than the child transaction if both are included in the same block.

This means that an unconfirmed transaction with a high feerate can incentivize miners to mine any of its ancestor transactions that are also unconfirmed. Nodes such as Bitcoin Core that implement such transaction selection policies for their block templates call this ancestor feerate mining. As long as a moderate percentage of miners implement ancestor feerate mining, wallets can use CPFP as a fee bumping technique.

Primary code and documentation

Mempool and mining in Bitcoin Core
Bitcoin Core #7600: ancestor feerate mining