A “domain extension” is slang for “Top Level Domain” or “country code Second Level Domain + country code Top Level Domain”.
In other words, if you buy “example.com” – “example” is your “base domain” and “.com” is your domain extension. In this case, your base domain is your “Second Level Domain” and your domain extension – “.com” is your TLD.
On the other hand, if you buy “example.co.uk” then “example” is still your base domain but “.co.uk” is your “domain extension”. In this case, your “base domain” is your “Third Level Domain”, and your “domain extension” – “.co.uk” – is your SLD + TLD.
