cotter vs cotter pin

cotter and cotter pin both are nouns.

Word	Noun	Adjective	Verb	Adverb
cotter	Yes	No	No	No
cotter pin	Yes	No	No	No

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

cotter: fastener consisting of a wedge or pin inserted through a slot to hold two other pieces together
cotter pin: a cotter consisting of a split pin that is secured (after passing through a hole) by splitting the ends apart

cotter (noun)	cotter pin (noun)
fastener consisting of a wedge or pin inserted through a slot to hold two other pieces together	a cotter consisting of a split pin that is secured (after passing through a hole) by splitting the ends apart
a medieval English villein
a peasant farmer in the Scottish Highlands