Source : WordNet®

unary
     adj : 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"

Source : Free On-Line Dictionary of Computing

unary
     
        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 same sequence as {nullary} and {binary}.
     
        2.  Base one.  A number base with only one
        digit, namely zero, and 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.
     
        (2001-02-25)