sing vs harmonize

sing and harmonize both are verbs.

Word	Noun	Adjective	Verb	Adverb
sing	No	No	Yes	No
harmonize	No	No	Yes	No

As verbs, harmonize is a hyponym of sing; that is, harmonize is a word with a more specific, narrower meaning than sing:

sing (verb)	harmonize (verb)
make a whining, ringing, or whistling sound	bring into consonance, harmony, or accord while making music or singing
divulge confidential information or secrets	bring into consonance or accord
to make melodious sounds	bring (several things) into consonance or relate harmoniously
produce tones with the voice	sing or play in harmony
deliver by singing	write a harmony for
	go together