Technically, any domain is a subdomain because it’s beneath another domain (even a top-level domain is beneath the root). However, in common usage, a subdomain refers to a domain at a level beyond that which is officially registered by the registrars of any given top level domain. For instance, in .uk, only third-level names are registered, under second-level names such as co.uk and org.uk, so example.org.uk isn’t generally called a “subdomain”, but test.example.org.uk is, since it’s not directly registered by a registrar but rather created by the owner of example.org.uk.
Since second-level names are directly registered in .com, anything at the third and higher level there is considered a subdomain. This includes example.uk.com, which is “registered” under uk.com, a domain that somebody registered and then proceeded to sell “registrations” of subdomains within, without any specific accreditation or approval by an Internet body such as ICANN.