The Collaborative International Dictionary of English v.0.48:

Monadic \Mo*nad"ic\, Monadical \Mo*nad"ic*al\, a.
   Of, pertaining to, or like, a monad, in any of its senses.
   See Monad, n. --Dr. H. More.
   [1913 Webster]

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

monadic

   1.  unary, when describing an operator or
   function.  The term is part of the dyadic, niladic
   sequence.

   2.  See monad.

   (1998-07-24)