What are the receive and fallback functions in Solidity?

Special functions that handle currency sent to a contract without a normal function call: receive() fires when the call data is empty, fallback() fires when data is sent but matches no function (or when there is no receive).

Neither uses the function keyword and both must be external payable. A common use is forwarding a plain transfer into a fund() function so the sender is still credited rather than the currency landing untracked.