A measure of the number and quality of links pointing to your website.
This is an 'off the page' factor in search engine optimisation, ie it has no reference to optimisation done to a webpage, such as text changes, TITLE or ALT tags.
The quality of a link is measured by where the link comes from, i.e. a link from a well established page with few outbound links will be worth far more than one from a poorly regarded page, and even more than a poorly regarded page with many outbound links.