METAPHYSICS

The branch of philosophy that systematically investigates first causes and the nature of ultimate reality.
KEYDICTIONARY

...the study of existence as such or, in Aristotle's words, of "being qua being" - the basic branch of philosophy.
(Ayn Rand, Philosophy: Who Needs It, p.3)

...the branch of philosophy that has as its subject matter the nature of the world, or of reality, rather than the nature of our knowledge, or of our language, or of our sciences about the world.
(Panayot Butchvarov, Being Qua Being, p.4)