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