What is Composition Of Matter?

Legal Definition
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.

Robinson on Patents has defined "composition of matter" in these terms:

A composition of matter is an instrument formed by the intermixture of two or more ingredients, and possessing properties which belong to none of these ingredients in their separate state. ...The intermixture of ingredients in a composition of matter may be produced by mechanical or chemical operations, and its result may be a compound substance resolvable into its constituent elements by mechanical processes, or a new substance which can be destroyed only by chemical analysis.

A newly synthesized chemical compound or molecule may be patented as a composition of matter. Patents have been allowed on transitory products, such as short-lived chemical intermediates.
-- Wikipedia
Legal Definition
In describing the subjects of patents, the Act of Congress of July 4, 1836, sect. 6, uses the words "composition of matter;" these words are usually applied to mixtures and chemical compositions, and in these cases it is enough that the compound is new. Both the composition and the mode of compounding may be considered as included in the invention, when the compound is new.
-- Bouviers Law Dictionary
Legal Definition
In patent law. A mixture or chemical combination of materials. Goodyear v. Railroad Co., 10 Fed. Cas. 664; Cahill v. Brown, 4 Fed. Cas. 1005; Jacobs v. Baker, 7 Wall. 295, 19 L. Ed. 200.
-- Black's Law Dictionary