Fan-in is the permissible number of digital input lines that can be connected to a logic, chip or other digital circuit without affecting the functionality of the circuit.
The number of input lines is limited because of the impedance load on the input circuit in question. For TTL logic, the fan-in is usually 1 or 2. Since there are almost always open emitter connections, the number of inputs in multi-emitter technology is dictated by the number of emitters. For output circuits, the number of digital output lines is determined by the fan-out.
Chip-to-chip interconnections, the fan-ins can be realized by means of wafer level packaging(WLP) as fan-in wafer level packaging( FI-WLP).