- it is used informally to mean a class of things, as in "the category of all living things"
- in philosophy:
- Aristotle classified everything into 10 famous categories
- others have tried the same, see categories of being
- Kant uses the word for the preconceived notions we use to organize sense data

- in mathematics:
- a collection of mathematical objects of the same kind, together with the structure-preserving maps 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; see Baire space.