length vs impermanency

length and impermanency both are nouns.

Word	Noun	Adjective	Verb	Adverb
length	Yes	No	No	No
impermanency	Yes	No	No	No

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

length (noun)	impermanency (noun)
a section of something that is long and narrow	the property of not existing for indefinitely long durations
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