A sidewide (often used in the plural ‘sitewides’) are links to your website that occur on every page of another website.  They are usually found in the footer, or some universal element of another website.  If you have 1,000 inbound links coming from one domain, then you have one sitewide.  These are not considered to be incredibly valuable, and are sometimes used as a blackhat tactic, and thus can be you penalized by search engines.