Fan-in

Fan-in is the number of inputs a logic gate can handle.

[1] For instance the fan-in for the AND gate shown in the figure is 3.

[3][4] Using logic gates with higher fan-in will help in reducing the depth of a logic circuit; this is because circuit design is realized by the target logic family at a digital level, meaning any large fan-in logic gates are simply the smaller fan-in gates chained together in series at a given depth to widen the circuit instead.

[5] In quantum logic gates the fan-in always has to be equal to the number of outputs, the fan-out.

Gates for which the numbers of inputs and outputs differ would not be reversible (unitary) and are therefore not allowed.