Web, domains and email
canonical
In short and in plain language
Canonical is a note in the code of a page.
It tells search engines which address is the main one. This helps when the same content is found at several addresses. That also goes for content that is very much alike.
Filters in a shop are a typical case.
Definition
Canonical is a note in the code of a page that tells search engines the preferred address. This helps when identical or very similar content is reachable under several addresses. Filters in an online shop are a typical case.