knuckleball vs knuckler

knuckleball and knuckler both are nouns.

Word	Noun	Adjective	Verb	Adverb
knuckleball	Yes	No	No	No
knuckler	Yes	No	No	No

As nouns, knuckleball and knuckler are synonyms defined as:

knuckleball and knuckler: a baseball pitch thrown with little speed or spin

knuckleball (noun)	knuckler (noun)
a baseball pitch thrown with little speed or spin	a baseball pitch thrown with little speed or spin