underscore vs emphasize

underscore is a noun but emphasize is not a noun.

Word	Noun	Adjective	Verb	Adverb
underscore	Yes	No	Yes	No
emphasize	No	No	Yes	No

As verbs, underscore and emphasize are synonyms defined as:

Other synonyms of underscore include emphasise, underline.

As verbs, emphasize is a hypernym of underscore; that is, emphasize is a word with a broader meaning than underscore:

Other hypernyms of underscore include accent, accentuate, emphasise, punctuate, stress.

underscore (noun)	emphasize (noun)
a line drawn underneath (especially under written matter)

underscore (verb)	emphasize (verb)
give extra weight to (a communication)	to stress, single out as important
draw a line or lines underneath to call attention to	give extra weight to (a communication)