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Metaphysics is the branch of philosophy investigating principles of reality transcending those of any particular science. Cosmology and ontology are traditional branches of metaphysics. It is concerned with explaining the ultimate nature of being and the world. "Metaphysician" is, counterintuitively, the name of someone that studies metaphysics, not metaphysicist.
The word derives from the Greek words (metá) (meaning "beyond" or "after") and (physiká) (meaning "physical"), "physical" referring to those works on matter by Aristotle in antiquity. The prefix meta- ("beyond") was attached to the chapters in Aristotle's work that physically followed after the chapters on "physics", in posthumously edited collections. Aristotle called some of the subjects treated there "first philosophy."
A central branch of metaphysics is ontology, the investigation into what types of things there are in the world and what relations these things bear to one another. The metaphysician also attempts to clarify the notions by which people understand the world, including existence, objecthood, property, space, time, causality, and possibility.
Before the development of modern science, scientific questions were addressed as a part of metaphysics known as "natural philosophy"; the term "science" itself meant "knowledge". The scientific method, however, made natural philosophy an empirical and experimental activity unlike the rest of philosophy, and by the end of the eighteenth century it had begun to be called "science" in order to distinguish it from philosophy. Thereafter, metaphysics became the philosophical enquiry of a non-empirical character into the nature of existence. Thus the original situation of metaphysics being integral with (Aristotelian) physics and science, has, in the West, become reversed so that scientists generally consider metaphysics antithetical to the empirical sciences.
History of metaphysics
One of the first metaphysicians is Parmenides of Elea. He held that the multiplicity of existing things, their changing forms and motion, are but an appearance of a single eternal reality (“Being”), thus giving rise to the Parmenidean principle that “all is one.” From this concept of Being, he went on to say that all claims of change or of non-Being are illogical. Because he introduced the method of basing claims about appearances on a logical concept of Being, he is considered one of the founders of metaphysics.
Metaphysics is called the "first philosophy" by Aristotle. The editor of his works, Andronicus of Rhodes, is thought to have placed the books on first philosophy right after another work, Physics, and called them (ta meta ta physika biblia) or, "the books that come after the [books on] physics." This was misread by Latin scholiasts, who thought it meant "the science of what is beyond the physical."  
Aristotle's Metaphysics was divided into three parts, in addition to some smaller sections related to a philosophical lexicon and some reprinted extracts from the Physics, which are now regarded as the proper branches of traditional Western metaphysics:
Ontology - The study of Being and existence; includes the definition and classification of entities, physical or mental, the nature of their properties, and the nature of change.
Natural Theology - The study of God; involves many topics, including among others the nature of religion and the world, existence of the divine, questions about Creation, and the numerous religious or spiritual issues that concern humankind in general.
Universal science - The study of first principles, which Aristotle believed to be the foundation of all other inquiries. An example of such a principle is the law of noncontradiction and the status it holds in non-paraconsistent logics.
Universal science or first philosophy treats of "being qua being" — that is, what is basic to all science before one adds the particular details of any one science. Essentially "being qua being" may be translated as "being insofar as being goes", or as, "being in terms of being". This includes topics such as causality, substance, species and elements, as well as the notions of relation, interaction, and finitude.
Metaphysics as a discipline was a central part of academic inquiry and scholarly education even before the age of Aristotle. Long considered "the Queen of Sciences", its issues were considered no less important than the other main formal subjects of physical science, medicine, mathematics, poetics and music. Since the beginning of modern philosophy during the seventeenth century, problems that were not originally considered within the bounds of metaphysics have been added to its purview, while other problems considered metaphysical for centuries are now typically relegated to their own separate regions in philosophy, such as philosophy of religion, philosophy of mind, philosophy of perception, philosophy of language, and philosophy of science.
In some cases, subjects of metaphysical scholarship have been found to be entirely physical and natural, thus making them part of physics proper (cf. Albert Einstein's Theory of Relativity).