An Etymological Dictionary of Astronomy and Astrophysics

1. Philo.: The doctrine, as opposed to idealism and materialism, that reality consists of two basic types of substance usually taken to be mind and matter or two basic types of entity, mental and physical.
   

   2. The state of being twofold or double. → dualism, → pluralism (Dictionary.com).

See also: → dual; → -ism.