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A wildcard Secure Socket Layer (SSL) certificate works in the same way as a regular SSL certificate in terms of securing the communication between the server and the client browser, but with the added bonus of being able to use the same certificate for the primary domain as well as all of its sub-domains. Its name may have been derived from the wildcard character that can be used to substitute for any character.
Wildcard SSL certificates use the Subject Alternative Names (SAN) field to secure all of the first-level sub-domains of a domain. For example, a single wildcard certificate provided for *.website.com is usable for mail.website.com, store.website.com and all other first-level sub-domains. By contrast, a regular SSL certificate can only be used for www.website.com.
The obvious benefit of this is cost savings, as a website owner only needs to pay for a single certificate instead of multiple regular certificates. Of course, this single wildcard certificate costs more than a regular certificate, and it only sees value if there are at least four to five sub-domains for it to be used on.