I always thought:

  - com, example.com, foo.example.com are all domains
  - com is a TLD
  - subdomain is a relative term, not an absolute one:
    . example.com is a subdomain of com
    . foo.example.com is a subdomain of example.com
    . bar.foo.example.com is a subdomain of foo.example.com

Yup, you’re correct. But in common usage, it would be weird to refer to example.com as a subdomain. Depending on the context, it would also be weird to refer to foo.example.com as a domain instead of a subdomain.

If somebody asked me what domain you’re using and you said “com”, you would technically have answered accurately but they’d be confused.