length vs light time

length and light time both are nouns.

Word	Noun	Adjective	Verb	Adverb
length	Yes	No	No	No
light time	Yes	No	No	No

As nouns, light time is a hyponym of length; that is, light time is a word with a more specific, narrower meaning than length:

length: size of the gap between two places
light time: distance measured in terms of the speed of light (or radio waves)

Other hyponyms of length include leg, arm's length, gauge, skip distance, wingspan, wingspread, wingspread, altitude.

length (noun)	light time (noun)
a section of something that is long and narrow	distance measured in terms of the speed of light (or radio waves)
the linear extent in space from one end to the other; the longest dimension of something that is fixed in place
continuance in time
the property of being the extent of something from beginning to end
size of the gap between two places