ensign vs national flag

ensign and national flag both are nouns.

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

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

ensign (noun)	national flag (noun)
colors flown by a ship to show its nationality	an emblem flown as a symbol of 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