Clean URL
A clean URL is short, readable, and stable: words separated by hyphens, a logical hierarchy, no technical identifiers or superfluous parameters. It reads like a summary of the page it points to.
In practice
site.com/glossary/internal-linking announces its content; site.com/index.php?id=4872&cat=12 announces nothing. Beyond readability, stability is paramount: every URL change requires a redirect and risks losing value. Slugs are frozen at publication — you can rename a title, never an address.