An opaque URL is a link that hides useful clues about its final destination, making it harder to see where it will really go at a glance. This can happen when a browser passes through a redirect, a tracking endpoint, or a heavily encoded parameter string before reaching the target site.
Opaque URLs matter in cyber security because people and security tools often depend on readable domains, subdomains, and paths to judge trust. Attackers can use this obscurity to disguise phishing pages, mask malicious redirects, or make a link look routine until after the click. Defenders reduce the risk by expanding redirects, decoding parameters, checking the real destination, and warning users when a link is difficult to verify.