Power domain
Beginner
A part of the chip that can be powered or slowed down separately from the rest.
Novice
A region of the chip with its own power supply that can be switched off (power gating) or run at its own voltage. Signals leaving a region that may be off need isolation cells to hold them at a safe value; signals between different voltages need level shifters.
Expert
Captured as power intent in a UPF file (IEEE 1801) and laid out as a voltage area. Each domain adds isolation, level shifters, retention registers, power switches and more power states to verify, so the number of domains is an architectural cost.
Explained in Architecture (Design Flow).
See also: power gating, DVFS, Voltage area (power-domain area).