Meaning

noun an unchanging intonation

noun a single tone repeated with different words or different rhythms (especially in rendering liturgical texts)

adjective of a sequence or function; consistently increasing and never decreasing or consistently decreasing and never increasing in value

originFrom the post-Classical Latin monotonus (“unvarying in tone”) or its etymon the Ancient Greek μονότονος (monótonos, “steady”, “unwavering”); compare cognate adjectives, namely the French monotone, the German monoton, the Italian monotono, and the Spanish monótono, as well as the slightly earlier English noun monotony and adjective monotonical.

Etymology from Wiktionary, CC BY-SA 3.0