WHOIS

A protocol and set of public registration data whose format and availability depend on the registry and zone policy.

In plain words

WHOIS shows who registered a domain, when, and with which registrar — unless the data is hidden by a privacy service.

Practical example

Using WHOIS you can learn a domain's expiry date and registrar to renew or transfer it in time.

Where you encounter it

When checking whether a domain is available, who owns it, and when registration expires.