SGML vs markup language

SGML and markup language both are nouns.

Word	Noun	Adjective	Verb	Adverb
SGML	Yes	No	No	No
markup language	Yes	No	No	No

As nouns, markup language is a hypernym of SGML; that is, markup language is a word with a broader meaning than SGML:

SGML: (computer science) a standardized language for the descriptive markup of documents; a set of rules for using whatever markup vocabulary is adopted
markup language: a set of symbols and rules for their use when doing a markup of a document

SGML (noun)	markup language (noun)
(computer science) a standardized language for the descriptive markup of documents; a set of rules for using whatever markup vocabulary is adopted	a set of symbols and rules for their use when doing a markup of a document