space heater vs heater

space heater and heater both are nouns.

Word	Noun	Adjective	Verb	Adverb
space heater	Yes	No	No	No
heater	Yes	No	No	No

As nouns, heater is a hypernym of space heater; that is, heater is a word with a broader meaning than space heater:

space heater: heater consisting of a self-contained (usually portable) unit to warm a room
heater: device that heats water or supplies warmth to a room

Other hypernyms of space heater include warmer.

space heater (noun)	heater (noun)
heater consisting of a self-contained (usually portable) unit to warm a room	(baseball) a pitch thrown with maximum velocity
	device that heats water or supplies warmth to a room