Thing is a matter of concern;
    the aim of effort or activity.
          

Imaginary is existing only in imagination;
    formed or characterized imaginatively;
    of or relating to the imaginary unit.
          

Real is having objective independent existence;
    capable of being detected;
    of or relating to real numbers.
          

Complex is a whole consisting of interrelated parts;
    typically involving Real and Imaginary aspects;
    hard to separate, analyze, solve;
    of or relating to complex numbers.
          

Ontology is metaphysics concerned with the nature and
        relations of being;
    a particular theory about the nature of being or the
        kinds of things that have existence;