second lieutenant vs 2nd lieutenant

second lieutenant and 2nd lieutenant both are nouns.

Word	Noun	Adjective	Verb	Adverb
second lieutenant	Yes	No	No	No
2nd lieutenant	Yes	No	No	No

As nouns, second lieutenant and 2nd lieutenant are synonyms defined as:

second lieutenant and 2nd lieutenant: a commissioned officer in the Army or Air Force or Marine Corps holding the lowest rank

second lieutenant (noun)	2nd lieutenant (noun)
a commissioned officer in the Army or Air Force or Marine Corps holding the lowest rank	a commissioned officer in the Army or Air Force or Marine Corps holding the lowest rank