What is a Multi-domain or SAN certificate?

Multi-domain or SAN (Secure Alternate Name) SSL certificates can cover multiple domain names on just one certificate. For example, Symantec and Thawte multi-domain certificates can cover up to 25 domains, whereas Comodo certificates can cover up to 250 domains with just a single SSL certificate. GeoTrust multi-domain certificates can cover anywhere from 25 to 250 domains, depending on the type of certificate you order.