WordNet (r) 3.0 (2006):

unary
    adj 1: consisting of or involving a single element or component;
           "in a unary operation in a mathematical system one
           element is used to yield a single result"

The Free On-line Dictionary of Computing (19 January 2023):

unary
unary operator

   1.  (or "monadic") A description of a
   function or operator which takes one argument, e.g. the
   unary minus operator which negates its argument.

   The term is part of the sequence nullary, unary, binary,
   ternary describing functions of zero to three arguments.

   2.  Base one.  A number base with only one
   digit, namely zero, which can therefore only be used to
   express the number zero.  Attempting to add one to zero
   results in an infinite sequence of carries.  Numbers in unary
   notation can be represented particularly efficiently however
   since each digit requires no storage.

   (2019-07-15)