Using the word Ontology in the way that metadata managers and programmers have been using it is pretty controversial. It always annoys people when words that have a specific meaning are co-opted by another group for another purpose. Think of the controversy over "gay". The jury is still out as to whether the expansion of the word "ontology" is a pejoration or a semantic shift

When I got my first real ontology gig, it was, of course, for an up-and-coming dotcom. I helped my boss place an ad in a few places, and because everything was fast and loose and the dotcom hadn't gotten around to hiring an HR department, we used my email address as the return address for prospective candidates. Some grad student from a philosophy department sent me a long rant about how I was personally responsible for the dumbing down of America (which, considering what the Web has done to our attention spans, may have some merit) and that overall we were evil people. He then went on to ask, rhetorically, "What IS an ontologist, anyway?"

I am afraid I emailed him back a rather snotty reply: "An ontologist is a librarian with stock options. Are you applying for the position?" I probably would take that email back if I could. On the other hand, if that's the snottiest I got during the dotcom boom, maybe I'm ahead of most.

Everyone except me has very specific ideas about what "Ontology" is. This guy gets squicky over the use of the definite article with "ontology". He says:

"If you want to really irritate me, refer to a plain taxonomic categorization as "an ontology". It's like calling a case of canned alphabet soup "a literature".

This is part of a general social trend that appropriates the most obvious outward manifestation of something they don't understand and devalues the original concept."

Whoa, hey, back off man. First off, I agree that taxonomy is only a probable component of ontology. I would disagree that taxonomy is "plain." A good taxonomy can be very complex and have multiple relationships within the hierarchy. The hierarchy itself imparts meaning to the concepts. (He goes on to make some very excellent points about the limitations of taxonomy and category structures as a representation of knowledge, so run off and read his post when you get the chance and can overcome his rather condescending writing style.)

Another comparison of the definitions of taxonomy and ontology occurs in the book Ontological Engineering : with examples from the areas of Knowledge Management, e-Commerce and the Semantic Web by Asuncion Gomez-Perez, et al. They say, "Sometimes the notion of ontology is diluted, in the sense that taxonomies are considered full ontologies...the ontology community distinguishes ontologies that are mainly taxonomies from ontologies that model the domain in a deeper way and provide _more restrictions_ on domain semantics. The community calls them lightweight ontologies and heavyweight ontologies respectively. On the one hand, lightweight ontologies [taxonomies] include concepts, concept taxonomies, relationships between concepts, and properties that describe concepts. On the other hand, heavyweight ontologies add axioms and constraints to lightweight ontologies. Axioms and constraints clarify the intended meaning of the terms gathered in the ontology."

I prefer the more relaxed approach of Heavyweight and Lightweight ontologies as opposed to the Philosophy community's stance that the word is being abused and the position of some others that ontologies and taxonomies have nothing to do with each other and anyone who uses the terms interchangeably is automatically an idiot trying to annoy ontological purists.