An expired domain is a domain name that was not renewed and returned to the public registration pool. Once that happens, a new party can register it and control the DNS records, email routing, and web content associated with the name. The original owner may still have old links, bookmarks, certificates, or code pointing at it.
In cybersecurity, expired domains matter because they can break services or be reused in abuse. Attackers may register them to capture traffic from hard-coded URLs, receive misdirected email, or impersonate a trusted brand or dependency. Defenders reduce the risk by renewing critical domains early, removing stale references from code and documentation, and monitoring for expired names that still appear in applications, scripts, or DNS records.