IFPGA

iFPGAs are Apple's internal field-programmable gate arrays and are used for prototyping new device models before manufacturing.

FastSim
A variant of iFPGA is called FastSim, which is likely derived from "Fast Simulation". References to the "FastSim" name are scarce online, but one possible match for Apple's use of it is http://pages.cs.wisc.edu/~wwt/fastsim/index.html, which is a micro-architecture simulator.

List of Known iFPGAs
iFPGA devices with "sim" in the name are FastSim.