Difference between A record and Subdomain

So, I'm curious about the difference between a subdomain and an A record in DNS. I've never had any issues, but I'm curious about how/why "A record" and "Subdomain" seem to be interchangeable for some people.

For example,

example.com (domain)

www.example.com (www is an "A" record within example.com)

server.america.example.com (server is an "A" record within the subdomain of "america.example.com).

This makes sense to me. However, I'm finding some people who argue that in the example "www.example.com", that www is actually a subdomain and not an A record.

Is there actually a difference between the two or is the difference simply in implementation?

Thanks,
Tim