The word category (plural categories; from Greek κατηγορια meaning "assertion" or "accusation", hence "categorical denial") has several meanings:

• it is used informally to mean a class of things, as in "the category of all living things". See categorization.
• in philosophy:
  • notably, Aristotle classified all things into 10 categories
  • others have made similar classifications, see categories of being
  • Kant uses the word for the preconceived notions we use to organize sense data
• in information technology, a category is a container for the names of things.
• in mathematics:
  • a category is a collection of mathematical objects of the same kind, together with the structure-preserving processes between them; as in "the category of all topological spaces and continuous maps". These are studied in category theory.
  • in topology, one distinguishes between sets of first and second category.
• The game, Categories
• on Wikipedia: see Wikipedia:Categorization for more about WP categories.