olemus
Internetis:
suurema domeeni osa
hierarhilises domeeninimede süsteemis,
kus ainult juurdomeen ei ole alamdomeen

Wiktionary:
2. (computing, Internet) a domain name that has been prefaced with additional parts separated with periods
3. (computing, Internet) an additional part prefaced to a domain name
In en.wiktionary.org, en is a subdomain.

ülevaateid
https://en.wikipedia.org/wiki/Subdomain

https://www.wpbeginner.com/glossary/subdomain/

https://blog.hubspot.com/website/what-is-a-subdomain