Synonyms for Monotonic (same or very similar meaning)
WordNet sense 1 (of a sequence or function; consistently increasing and never decreasing or consistently decreasing and never increasing in value):
monotoneWordNet sense 2 (sounded or spoken in a tone unvarying in pitch):
flat, monotonousWordNet sense 3 (characterized by lack of variation in pitch, tone, or volume):
unmodulatedFrom the ODE community, based on WordNetadd/edit