Use WordNet. Together with the associated relationship information you can get for WordNet in RDF format you would get the following information for 'polar bear':
wn30:synset-ice_bear-noun-1
white bear of Arctic regions
Synonyms Thalarctos maritimus, Ursus Maritimus, ice bear
--type--> wn30:synset-bear-noun-1
--memberMeronymOf--> wn30:synset-Thalarctos-noun-1
Following the Type relation further would also reveal that it's a Mammal, and an Animal
One way to try Wordnet is on my the demo page for my Natural Language Engine. Simply type 'define' and then the word you want to look up. (P.S. It may take a few seconds to restart if nobody has been there recently.)