Physicalism is the thesis that everything is physical, or as contemporary philosophers sometimes put it, that everything supervenes on the physical. The thesis is usually intended as a metaphysical thesis, parallel to the ancient Greek philosopher Thales’s thesis that everything is water, or the idealism of the 18th Century philosopher Berkeley, that everything is mental. The general idea is that the nature of the actual world (i.e. the universe and everything in it) conforms to a certain condition, the condition of being physical. Of course, physicalists don’t deny that the world might contain many items that at first glance don’t seem physical — items of a biological, or psychological, or moral, or social nature. But they insist nevertheless that at the end of the day such items are wholly physical.
Physicalism is sometimes known as materialism. Historically, materialists held that everything was matter — where matter was conceived as “an inert, senseless substance, in which extension, figure, and motion do actually subsist” (Berkeley, Principles of Human Knowledge, par. 9). The reason for speaking of physicalism rather than materialism is to abstract away from this historical notion, which is usually thought of as too restrictive — for example, forces such as gravity are physical but it is not clear that they are material in the traditional sense (Dijksterhuis 1961, Yolton 1983). It is also to emphasize a connection to physics and the physical sciences. Indeed, physicalism is unusual among metaphysical doctrines in being associated historically with a commitment both to the sciences and to a particular branch of science, namely physics.
Given that physicalism, and naturalism more generally, is the reigning orthodoxy today in higher education and in large pockets of our culture, it’s helpful to keep in mind that none of these worldviews are verifiable by science, or deducible from science. Instead, these are philosophical approaches to science, which will have to gain their justification from philosophical arguments. Many today mistakenly believe that physicalism (and naturalism, scientism, etc.) just somehow immediately follows from the scientific method or experiments in a laboratory. But, that’s a misunderstanding, and quite false.