1. Tediously repetitious or lacking in variety
   "nothing is so monotonous as the sea";
   - humdrum
    
2. Sounded or spoken in a tone unvarying in pitch
   "the owl's faint monotonous hooting";
   - flat, monotone, monotonic
    
3. (mathematics) of a sequence or function; consistently increasing and never decreasing or consistently decreasing and never increasing in value
   "The monotonous decay of a radioactive isotope follows an exponential curve";
   - monotonic, monotone

See also: decreasing monotonic, dull, hebetudinous [rare], increasing monotonic, unmodulated

Encyclopedia: Monotonous