Vocabularies to classify terms

Dear all, I am searching for vocabularies that let me classify terms,
something similar to an upper ontology.I am explaining that with a term
tree. Lets imagine that we have the first node that we will called
"Categories". Then we will hace the main categories in the second level of
the tree. And following we will have subcategories or terms. If a look for
a term it will be classified by its main category.

1. Categories
    1.1 Biology
          1.1.1 Cell
          1.1.2 Protein
          ...
          1.1.N Mineral
    1.2 Mathematics
          1.2.1 Equation
           ...
          1.2.N Geometry
                    1.2.N.1 Triangle
     ....
     1.N Physics
            1.N.1 Gravity

So in this case if a want to classify the term gravity, the answer will be
Physics. If I want to classify triangle, the answer will be Mathematics.

Does anyone knows an ontology, taxonomy, etc similar to this. I need it to
be expressed as rdf, owl or similar.
Thanks.