step ladder vs ladder

step ladder and ladder both are nouns.

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

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

step ladder: a folding portable ladder hinged at the top
ladder: steps consisting of two parallel members connected by rungs; for climbing up or down

step ladder (noun)	ladder (noun)
a folding portable ladder hinged at the top	steps consisting of two parallel members connected by rungs; for climbing up or down
	a row of unravelled stitches
	ascending stages by which somebody or something can progress

step ladder (verb)	ladder (verb)
	come unraveled or undone as if by snagging