step stool vs stool

step stool and stool both are nouns.

Word	Noun	Adjective	Verb	Adverb
step stool	Yes	No	No	No
stool	Yes	No	Yes	No

As nouns, stool is a hypernym of step stool; that is, stool is a word with a broader meaning than step stool:

step stool (noun)	stool (noun)
a stool that has one or two steps that fold under the seat	a simple seat without a back or arms
	a plumbing fixture for defecation and urination
	(forestry) the stump of a tree that has been felled or headed for the production of saplings
	solid excretory product evacuated from the bowels

step stool (verb)	stool (verb)
	have a bowel movement
	grow shoots in the form of stools or tillers
	react to a decoy, of wildfowl
	lure with a stool, as of wild fowl