Function selector
The first four bytes identifying a function in a standard Ethereum ABI-encoded call.
In more detail
The selector is derived from a function's name and parameter types, telling the contract which function is being requested. It isn't the full request: the arguments follow it in calldata. Four bytes also aren't a universal guarantee of uniqueness, so decoding should use the intended contract's actual interface.
How you might use it
A transfer(address,uint256) call starts with its selector, followed by the encoded recipient address and token amount.
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