charley horse vs spasm

charley horse and spasm both are nouns.

Word	Noun	Adjective	Verb	Adverb
charley horse	Yes	No	No	No
spasm	Yes	No	No	No

As nouns, spasm is a hypernym of charley horse; that is, spasm is a word with a broader meaning than charley horse:

charley horse: a muscular cramp (especially in the thigh or calf) following vigorous exercise
spasm: a painful and involuntary muscular contraction

Other hypernyms of charley horse include cramp, muscle spasm.

charley horse (noun)	spasm (noun)
a muscular cramp (especially in the thigh or calf) following vigorous exercise	(pathology) sudden constriction of a hollow organ (as a blood vessel)
	a painful and involuntary muscular contraction