constancy vs monotony

constancy and monotony both are nouns.

Word	Noun	Adjective	Verb	Adverb
constancy	Yes	No	No	No
monotony	Yes	No	No	No

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

Other hyponyms of constancy include invariance, metastability.

constancy (noun)	monotony (noun)
the quality of being enduring and free from change or variation	the quality of wearisome constancy, routine, and lack of variety
faithfulness and dependability in personal attachments (especially sexual fidelity)	constancy of tone or pitch or inflection
(psychology) the tendency for perceived objects to give rise to very similar perceptual experiences in spite of wide variations in the conditions of observation