A "public suffix" is a domain name under which Internet users can directly register own names.

Browsers and other web clients can use it to avoid privacy-leaking "supercookies", avoid privacy-leaking "super domain" certificates, domain highlighting parts of the domain in a user interface, and sorting domain lists by site.

Libpsl has built-in PSL data for fast access, allowing to load PSL data from files, checks if a given domain is a public suffix, provides immediate cookie domain verification, finds the longest public part of a given domain, finds the shortest private part of a given domain, works with international domains (UTF-8 and IDNA2008 Punycode), is thread-safe, and handles IDNA2008 UTS#46.