Everything is a subdomain of its parent.
Hidden secret: technically every fully-qualified domain name ends with a dot ".", but you can safely leave it out. So "www.google.com" should be "www.google.com.". Sometimes you need the terminal dot to disambiguate relative domain names, but let's ignore that for now.
Each part of a domain name, separated by the dot, is a "label" - but you only see that term in the context of DNS server documentation, usually because there's a length-limit of 63 characters per label.
There is no standard nomenclature for the "human-identifiable" parts that you list, unfortunately, but the general terminology I've come across is:
"." - the root
".com" - (generic) top-level domain
"google" - (in some circles) top-level domain (as it's the top-level a consumer can register), otherwise a "subdomain of the dotcom TLD"
"www" - a subdomain of "google.com"
"something.www.google.com" - "a subdomain of google.com, the subdomain being "something.www"
Now technically, a label is not necessarily a "subdomain" if it's just a CNAME or A record, but this is an implementation detail. Technically a label could only refer to a true subdomain if it has its own SOA record, but that's a semantic debate.
Overall? I wouldn't worry. People will generally know what you mean when you say "subdomain", even if a subdomain is split over more than one label.