In United States patent law, a composition of matter is one of the four principal categories of things that may be patented. The other three are a process (also termed a method), a machine, and an article of manufacture. In United States patent law, that same terminology has been in use since the first patent act in 1790 (with the exception that processes were formerly termed "arts").
The United States Supreme Court has defined "composition of matter" to mean "all compositions of two or more substances and all composite articles, whether they be the results of chemical union, or of mechanical mixture, or whether they be gases, fluids, powders or solids." That definition is problematic, however, because composite articles can be articles of manufacture—as in the case of a piece of plywood, a concrete sidewalk, a road, a fibreglass bathtub, a (kitchen) countertop, or a flitch beam.
