What is an Ontology?

While reading about Semantic Web we come across one common term always, The Ontology. What I found that every site and author gives a different definition of ontology. The best of that I liked was what was given by Tom Gruber "An ontology is a specification of a conceptualization".

I tried to find the answer at few more places as what the ontology is all about. There were few interesting answers I got:

Webster Dictionary
1 : a branch of metaphysics concerned with the nature and relations of being
2 : a particular theory about the nature of being or the kinds of things that have existence

David Koepsell, Center for Commercial Ontology:

“Ontology is the very first science. Ontology involves discovering categories and fitting objects into them in ways that make sense…. When we make a list of things to do, or of records and books we most want to buy, or videos we intend to rent, we are categorizing-we are engaging in rudimentary ontology. By prioritizing items in a list, we are assigning relationships among various things. Ontology can be relatively simple, or it can be quite complex.

Ontology becomes more complex, and even daunting, when we begin to grapple with large domains of objects with complex relationships among them. For instance, anyone who has attempted to outline the processes and components of even a relatively small enterprise has experienced the brain-cramps that can come with complex ontology.”

Wordnet from Princeton University defines ontology as
1. (computer science) a rigorous and exhaustive organization of some knowledge domain that is usually hierarchical and contains all the relevant entities and their relations
2. the metaphysical study of the nature of being and existence

There are many more definitions available on net and one is bound to get confused when he/she comes across so many different kinds of definitions of the same word. But what actually the Ontology is? The question still remains unanswered or not convincingly answered.

From my study about ontology I found that It is explanation about the things as they exist in the real world. I am not after another definition here but just making an attempt to simplify for my understanding. But at the same time one of my associates think that Ontology is a Reasoning mechanism and it is representation of the logic.

What I am looking to work out is a generalized way to define the ontology which is universal in nature. I am sure many out there will disagree with me on this point that there could be a Universal Ontology. But my question to those people is why it can't be? What is so difficult about ontology which is stopping it from being represented in a general form.

In one of my earlier post on Knowledge Representation I mentioned that Knowledge Representation makes and ontological Commitment. If that is the case then each object in this world is part of one or more ontology. And if an object can exist in more than one ontology then there has to be some sort of similarity between two of them. Just thinking on those lines I guess it is possible to have a set of general ontology defined for all kinds of objects in existence today.

For those who are reading, I would love to know your view on this.

Until next time... :)