national flag vs ensign

national flag and ensign both are nouns.

Word	Noun	Adjective	Verb	Adverb
national flag	Yes	No	No	No
ensign	Yes	No	No	No

As nouns, national flag and ensign are synonyms defined as:

national flag (noun)	ensign (noun)
an emblem flown as a symbol of nationality	colors flown by a ship to show its nationality
	an emblem flown as a symbol of nationality
	a person who holds a commissioned rank in the United States Navy or the United States Coast Guard; below lieutenant junior grade