SAN (Subject Alternative Name)
Subject Alternative Name is the certificate field listing every hostname the
certificate covers. A certificate for example.com and www.example.com has
both in its SAN list.
The Common Name is dead
Certificates also carry a Common Name (CN), which is what people still quote when they talk about "the certificate's domain". Every current browser ignores it. Chrome stopped honouring CN in 2017; the others followed.
If a hostname is not in the SAN list, the certificate does not cover it, whatever the CN says.
Why certificates cover the wrong things
The two failures we see:
A missing www. The certificate covers example.com and not
www.example.com, or the reverse. Half your visitors get a full-page warning
depending on how they typed it.
A stale list. A certificate covering eight hostnames is reissued from a config that lists seven, and the eighth breaks — usually something low-traffic enough that nobody notices for weeks.
Related
The SSL certificate checker lists the SAN entries for any host. IT Watch checks that the certificate actually being served covers the domain it is serving, which is not the same as checking that it is valid.
See also: Wildcard certificate, OCSP stapling.