noun

• (philosophy, uncountable) The branch of philosophy which studies fundamental principles intended to describe or explain all that is, and which are not themselves explained by anything more fundamental; the study of first principles; the study of being insofar as it is being (Latin: ens in quantum ens).
• (philosophy, countable) The view or theory of a particular philosopher or school of thinkers concerning the first principles which describe or explain all that is.
• (logic, uncountable, by extension from the philosophical sense) The metalogic of physics; the logical framework of physics.
• (uncountable, by extension from the philosophical sense) Any fundamental principles or rules.
• (uncountable) The study of a supersensual realm or of phenomena which transcend the physical world.
• (uncountable, derogatory) Displeasingly abstruse, complex material on any subject.