Registry
The organization that operates a TLD's authoritative database — Verisign for .com/.net, Identity Digital for .io, Donuts for .app/.dev, etc. Sets the rules registrars must follow.
What the registry does
Maintains the authoritative database of who owns what under their TLD, runs the root nameservers for the TLD, and enforces policies (eligibility, premium pricing, disputes).
Why this matters to buyers
Registry policy ultimately wins — if .ai’s registry refuses a transfer pending a compliance review, no registrar can override that.