noun (n)

• a collection of things sharing a common attribute(noun.group)

  Synonym:
  class, family
  

  source: wordnet30
• a general concept that marks divisions or coordinations in a conceptual scheme(noun.cognition)

  source: wordnet30
• One of the highest classes to which the objects of knowledge or thought can be reduced, and by which they can be arranged in a system; an ultimate or undecomposable conception; a predicament.(noun)

  source: webster1913