(also wildcard SSL certificate)
A public key certificate that applies to multiple first-level subdomains of one domain, principally used to secure websites with HTTPS. Wildcard certificates contain an asterisk (the wildcard character) in the domain name field.
Google: Docs, Calendar, and many other Google services are secured with a wildcard certificate for *.google.com.
Facebook: Meta for Developers and other subdomains are secured with a wildcard certificate for *.facebook.com.