(n): a group of people or things arranged by class or category
(n): (physics) the property of matter and electromagnetic radiation that is characterized by the fact that some properties can be explained best by wave theory and others by particle theory
(n): a basic or essential attribute shared by all members of a class
(n): (geometry) the interchangeability of the roles of points and planes in the theorems of projective geometry
(n): the quality of being capable of exchange or interchange
