What is a canonical url?

Canonical URL defined as the preferred URL for the page content to be indexed in the Major Search Engines. Canonical URL is used by the Major Search Engines like Google, Yahoo, Bing, etc. Many websites have two or more links of the same content with different parameter.

Canonical URL is the URL that webmasters want the search engine to serve as the official webpage address. Or in other words, a canonical URL is the URL that webmasters want visitors to see.

A Canonical link element is an HTML element that helps webmasters prevent duplicate content issues by specifying the "canonical" or "preferred" version of a web page as part of search engine optimization

Cannonical is a process to redirect different url having same content to a single url.
ie. if you type domain.com it should be redirected to www.domain.com or vice versa. Where as both the url are having same content. So any search engine will not get confused as duplicate content.

 A canonical URL is the URL that you want visitors to see.