line of longitude vs date line

line of longitude and date line both are nouns.

Word	Noun	Adjective	Verb	Adverb
line of longitude	Yes	No	No	No
date line	Yes	No	No	No

line of longitude (noun)	date line (noun)
an imaginary great circle on the surface of the earth passing through the north and south poles at right angles to the equator	an imaginary line on the surface of the earth following (approximately) the 180th meridian