eFPGA (embedded FPGA)
Beginner
A small rewirable FPGA part built inside a custom chip, so that one corner of the chip can still change.
Novice
FPGA fabric licensed as a block (IP) and placed inside an ASIC or SoC. Most of the chip stays fixed and efficient; the eFPGA holds the part of the job that may change after tapeout.
Expert
Delivered as hard or soft IP with its own configuration interface and an FPGA tool flow. Costs fabric area at FPGA density for the volatile function only. Open generators such as FABulous use Yosys and nextpnr for the user flow.
Explained in Where FPGAs win (Architectures).
See also: FPGA, ASIC, IP block (silicon IP).