Canonical URL

A canonical URL is the single address you designate as the definitive version of a page when the same or very similar content is reachable through several URLs. Duplicate URLs arise easily — through tracking parameters, session IDs, print versions, or trailing-slash differences — and they split signals that search engines would otherwise consolidate onto one page. A canonical tag, placed in the page head, tells search engines which URL to treat as primary. Used correctly it consolidates ranking signals and avoids duplicate-content confusion; used carelessly, such as pointing many pages at one, it can hide pages you wanted indexed.