Registry
A registry is the organisation that centrally administers a top-level domain and sets the rules for registration, deletion and deadlines. Examples: DENIC (.de), nic.at (.at), EURid (.eu).
How it differs: the registry administers the TLD centrally; a registrar is the provider through which end users register.
Example: the deletion rules for .de — such as the 30-day RGP — are set by the registry DENIC.