Fan-in
Beginner
How many inputs a gate has.
Novice
The number of inputs to a gate. A NAND2 has fan-in 2. In CMOS, higher fan-in means taller transistor stacks, so gates are usually kept to four inputs or fewer.
Expert
Input count of a gate. Each extra input adds a series device to one network, raising logical effort (NAND: ) and parasitic delay ( by logical effort; growing faster, roughly as , by Elmore). Libraries stop around four inputs; wider functions are built as trees.