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;