What is a Bitcoin hard fork?

A Bitcoin hard fork is a radical change to the Bitcoin protocol that makes previously invalid blocks, transactions, and/or other data valid, or vice-versa. A Bitcoin hard fork requires all nodes running the new version of the Bitcoin software to upgrade in order to remain compatible with the network. This usually involves splitting the blockchain, creating two separate and incompatible blockchains. This is different from a soft fork, which is a change to the protocol that is compatible with the previous version of the software and does not require a split in the blockchain.