A wildcard certificate is a type of SSL/TLS certificate that can be used to secure multiple subdomains of a domain. It includes a wildcard character (*) in the domain name field, which allows it to secure all subdomains of the primary domain. For example, a wildcard certificate for *.example.com could be used to secure www.example.com, mail.example.com, store.example.com, or any other subdomain name in the example.com. Wildcard certificates are generally used in organizations with many subdomains. They can be purchased from any Certificate Authority (CA), and when ordering a wildcard certificate, you must specify the main domain name and the subdomains.

Wildcard certificates are categorized based on the validation level, the number of domains, and the number of servers they can be used with. They are named as domain validation wildcard certificate, organization validation wildcard certificate, and extended validation wildcard certificate when categorized according to validation level. The name Multi-domain wildcard certificates and Multi-server wildcard certificates are given according to the number of domains and servers.

Wildcard certificates can save time and money compared to managing individual certificates for subdomains. However, they have some limitations. For example, only a single level of subdomain matching is supported in accordance with RFC 2818. Additionally, if a certificate has to be revoked on one subdomain, it has to be revoked on all the others as well. Therefore, purchasing separate certificates may cost a bit more and require more administration, but it also ensures that each subdomain is individually protected.