orchid cactus vs cactus

orchid cactus and cactus both are nouns.

Word	Noun	Adjective	Verb	Adverb
orchid cactus	Yes	No	No	No
cactus	Yes	No	No	No

As nouns, cactus is a hypernym of orchid cactus; that is, cactus is a word with a broader meaning than orchid cactus:

orchid cactus: any cactus of the genus Epiphyllum having flattened jointed irregularly branching stems and showy tubular flowers
cactus: any succulent plant of the family Cactaceae native chiefly to arid regions of the New World and usually having spines

orchid cactus (noun)	cactus (noun)
any cactus of the genus Epiphyllum having flattened jointed irregularly branching stems and showy tubular flowers	any succulent plant of the family Cactaceae native chiefly to arid regions of the New World and usually having spines