"Metaphysics" is a word that 'Aristotle's editors invented to name books he wrote that came after his books on physics. These later books deal with the modes of being or existence, with the reality that is independent of our minds and is immaterial or nonphysical. Unlike mathematics, metaphysics does not deal with ideal objects abstracted from the real or physical things. It deals with the immaterial, such as God and the human intellect. Aristotle sometimes refers to it as theology today we would call it philosophical theology.
In the nineteenth century something called positivism arose in the writings of Auguste Comte. For him, only the positive empirical sciences give us knowledge of reality. For him, religion and philosophy were the gibberish of a bygone era.
Positivism was further developed in the twentieth century by the Viennese circle of antimetaphysical authors. It later became the position known as "logical positivism." Without any understanding of what was meant by calling the book "Metaphysics", the logical positivist dismissed metaphysical discussion as a misuse of language.
Today, another exclusively modern error stands in the way of reviving what Aristotle treated as a valid branch of philosophical thought. It is the error on ontological idealism -- the denial of any reality independent of our minds. This position, of course, invalidates philosophy itself as a first-order discipline.