lieutenant JG vs lieutenant junior grade

lieutenant JG and lieutenant junior grade both are nouns.

Word	Noun	Adjective	Verb	Adverb
lieutenant JG	Yes	No	No	No
lieutenant junior grade	Yes	No	No	No

As nouns, lieutenant JG and lieutenant junior grade are synonyms defined as:

lieutenant JG and lieutenant junior grade: an officer holding a commissioned rank in the United States Navy or United States Coast Guard; below lieutenant and above ensign

lieutenant JG (noun)	lieutenant junior grade (noun)
an officer holding a commissioned rank in the United States Navy or United States Coast Guard; below lieutenant and above ensign	an officer holding a commissioned rank in the United States Navy or United States Coast Guard; below lieutenant and above ensign