Can a Second-level domain be considered as a Fully-qualified domain?

I often see people talking about FQDNs (fully-qualified domains) to describe subdomains (e.g., networkengineering.stackexchange.com) that are under a second-level domain (e.g., stackexchange.com). I wonder if a SLD can be considered a FQDN itself. For instance, in this case, stackexchange.com is already enough to bring us to the front page of Stack Exchange. Is there any conventional document/standard behind these terminologies?

Yes.

Any domain is a subdomain and is an FQDN, it all depend on the context, you can not say anything just by looking at the string.

From RFC 8499 DNS Terminology:

Domain name: An ordered list of one or more labels.

and

Fully-Qualified Domain Name (FQDN): This is often just a clear way of saying the same thing as "domain name of a node", as outlined above. However, the term is ambiguous. Strictly speaking, a fully-qualified domain name would include every label, including the zero-length label of the root: such a name would be written "www.example.net." (note the terminating dot). But, because every name eventually shares the common root, names are often written relative to the root (such as "www.example.net") and are still called "fully qualified".

and

Subdomain: "A domain is a subdomain of another domain if it is contained within that domain. This relationship can be tested by seeing if the subdomain's name ends with the containing domain's name."