noun (n)

• the act of distributing things into classes or categories of the same type(noun.act)

  Synonym:
  assortment, categorisation, categorization, compartmentalisation, compartmentalization
  

  source: wordnet30
• a group of people or things arranged by class or category(noun.group)

  Synonym:
  categorisation, categorization
  

  source: wordnet30
• the basic cognitive process of arranging into classes or categories(noun.cognition)

  Synonym:
  categorisation, categorization, sorting
  

  source: wordnet30
• restriction imposed by the government on documents or weapons that are available only to certain authorized people(noun.act)

  Antonym:
  declassification
  

  source: wordnet30
• The act of forming into a class or classes; a distribution into groups, as classes, orders, families, etc., according to some common relations or affinities.(noun)

  source: webster1913