hot pad vs heater

hot pad and heater both are nouns.

Word	Noun	Adjective	Verb	Adverb
hot pad	Yes	No	No	No
heater	Yes	No	No	No

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

hot pad: heater consisting of electrical heating elements contained in a flexible pad
heater: device that heats water or supplies warmth to a room

Other hypernyms of hot pad include warmer.

hot pad (noun)	heater (noun)
heater consisting of electrical heating elements contained in a flexible pad	(baseball) a pitch thrown with maximum velocity
a pad for use under a hot dish to protect a table	device that heats water or supplies warmth to a room