Lot of information scientist use the term "ontology" without really having a clear understanding of what it means. The following blogpost by Dr. Marshall X Ma of Rensselaer Polytechnic Institute gives a holistic definition of "ontology":

http://tw.rpi.edu/weblog/2013/12/19/what-is-ontology/

 

in short, ontology is:

  • For fun: the invisible hand behind anything;
  • In philosophy: (uncountable) the science or study of being; that branch of metaphysics concerned with the nature or essence of being or existence; (countable) a theory or conception relating to the nature of being;
  • In computer science: shared conceptualization of domain knowledge.