charley-horse vs muscle spasm

charley-horse and muscle spasm both are nouns.

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

As nouns, muscle spasm is a hypernym of charley-horse; that is, muscle 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
muscle spasm: a painful and involuntary muscular contraction

Other hypernyms of charley-horse include cramp, spasm.

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