step ladder vs stepladder

step ladder and stepladder both are nouns.

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

As nouns, step ladder and stepladder are synonyms defined as:

step ladder (noun)	stepladder (noun)
a folding portable ladder hinged at the top	a folding portable ladder hinged at the top