I have still this question that keeps bugging me. If we call .com, .com.au, .co.uk, etc. TLDs or Top Level Domains. What do we call or what is the correct term for the www and non-www? Note: I am not quite sure if "Domain Names" is the best section for this topic, I hardly can't find the right section for this one. But anyway, If you know the right section, please advice. I am willing to remove this post and transfer this to the correct one.
Technically, that part of a URL is the subdomain. You could have anything here, www/www2/mail/example/images.domain.com. Whether you'd call it that when trying to explain to someone what you mean by www or non-www is something else and probably depends on the other persons level of understanding on the topic.
A url without a subdomain is called non-canonical these guys have subdomains www.mysite.com admin.mysite.com promos.mysite.com and this one doesn't mysite.com Most people would (quite reasonably) expect www.mysite.com to be the same as mysite.com but, technically, you can serve up completely different websites. You wouldn't want to, but you could.