Fanout
Fanout turns up in 2 fields: PCBs, electronics. Here is what it means to the people in each one.
Fanout in PCBs
Short connections that bring each pin of a dense part out to a via so it can be routed on other layers.
Fanout is the first stage of routing a dense part: each pad gets a short trace and a via, or a via in the pad, so its signal can drop to an inner layer. For BGAs it is planned ring by ring, with outer balls routed on the surface and inner balls taken down through vias to successive layers, and the via grid sets how many layers the escape needs. A clean fanout makes the rest of the routing possible.
"Do the fanout on the FPGA first and lock it, then we'll know how many signal layers we really need."
Fanout in electronics
The number of subsequent logic gate inputs that a single output can reliably drive.
The maximum number of standard digital inputs that the output of a logic gate can feed while maintaining valid voltage levels. Driving too many inputs draws excessive current, which degrades the logic high or low voltage and slows down the transition times due to accumulated input capacitance. If a signal must drive a massive number of destinations, designers insert buffer trees to multiply the driving capability.
"The clock signal has a massive fanout across the board, so we need to route it through a clock distribution buffer."