rheostat vs dimmer

rheostat and dimmer both are nouns.

Word	Noun	Adjective	Verb	Adverb
rheostat	Yes	No	No	No
dimmer	Yes	No	No	No

As nouns, dimmer is a hyponym of rheostat; that is, dimmer is a word with a more specific, narrower meaning than rheostat:

rheostat: resistor for regulating current
dimmer: a rheostat that varies the current through an electric light in order to control the level of illumination

rheostat (noun)	dimmer (noun)
resistor for regulating current	a rheostat that varies the current through an electric light in order to control the level of illumination