culture medium vs agar

culture medium and agar both are nouns.

Word	Noun	Adjective	Verb	Adverb
culture medium	Yes	No	No	No
agar	Yes	No	No	No

As nouns, agar is a hyponym of culture medium; that is, agar is a word with a more specific, narrower meaning than culture medium:

culture medium: (bacteriology) a nutrient substance (solid or liquid) that is used to cultivate micro-organisms
agar: any culture medium that uses agar as the gelling agent

Other hyponyms of culture medium include nutrient agar.

culture medium (noun)	agar (noun)
(bacteriology) a nutrient substance (solid or liquid) that is used to cultivate micro-organisms	a colloidal extract of algae; used especially in culture media and as a gelling agent in foods
	any culture medium that uses agar as the gelling agent