Physical Design
Fan-In
The number of inputs to a logic gate, affecting its propagation delay and drive capability.
Detailed Explanation
High fan-in gates (many inputs) are slower than low fan-in equivalents because more transistors must switch. Synthesis tools may decompose high fan-in operations into trees of smaller gates for better timing.
Fan-in limits exist in standard cell libraries—a library might offer 2-input and 4-input gates but not 8-input. Logic requiring more inputs is built from available gates.