FPGA
Beginner
A chip that can be rewired with new instructions after it is built.
Novice
Field-programmable gate array: a chip made of many small configurable logic blocks and programmable wiring. A configuration file (the bitstream) sets what it does, so the function can change without making a new chip.
Expert
Avoids the up-front cost of masks and allows updates in the field, at a cost in power, area and speed against a custom ASIC. It is still a commercial part with its own supply chain, and the bitstream becomes an asset to protect.
Explained in Defense (Industries).