The Collaborative International Dictionary of English v.0.48:

Monotonic \Mon`o*ton"ic\, Monotonical \Mon`o*ton"ic*al\, a.
   1. Of, pertaining to, or uttered in, a monotone; monotonous.
      "Monotonical declamation." --Chesterfield.
      [1913 Webster]

   2. (Math.) Always increasing or always decreasing, as the
      value of the independent variable increases; -- of a
      function.
      [PJC]

WordNet (r) 3.0 (2006):

monotonic
    adj 1: of a sequence or function; consistently increasing and
           never decreasing or consistently decreasing and never
           increasing in value [syn: monotonic, monotone] [ant:
           nonmonotonic]
    2: sounded or spoken in a tone unvarying in pitch; "the owl's
       faint monotonous hooting" [syn: flat, monotone,
       monotonic, monotonous]

The Free On-line Dictionary of Computing (30 December 2018):

monotonic

   In domain theory, a function f : D -> C is monotonic (or
   monotone) if

   	for all x,y in D,  x <= y  =>  f(x) <= f(y).

   ("<=" is written in LaTeX as \sqsubseteq).

   (1994-11-24)