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