Definition
A canonical URL is the operator-declared preferred address for a page when multiple URLs may serve the same content (https vs http, www vs apex, tracking params, locale variants). Declared via <link rel=canonical href='…'> in the page head, or via an HTTP header. Without it, AI engines may treat duplicates as separate documents, splitting citations across both and downranking each.
<link rel=canonical href='https://example.com/article' /> placed on both /article and /article?utm_source=twitter so engines unify them.
How to optimize
Self-canonical on every primary page. For paginated lists, canonical to the list root or use rel='next/prev'. Audit with VectorCite's canonical signal.