A wildcard certificate is a digital certificate that is applied to a domain and all its subdomains.
Wildcard notation consists of an asterisk and a period before the domain name. Secure Sockets Layer (SSL) certificates often use wildcards to extend SSL encryption to subdomains. A conventional SSL certificate works on a single domain, for example www.domain.com. A wildcard certificate for *.domain.com will also protect mail.domain.com, vpn.domain.com, payment.domain.com, etc.
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Extending a single certificate to subdomains rather than purchasing separate certificates can save money and make administration easier. The disadvantage, however, is that if a certificate has to be revoked on one subdomain, it has to be revoked on all the others as well. If a subdomain like payment.domain.com is compromised, for example, so are the mail and vpn subdomains. Purchasing separate certificates may cost a bit more and require more adminstration, but it also ensures that each subdomain is individually protected.
See also: SSL VPN