What is formed by the chemical union of two or more elements in definite proportions by weight?

A compound is formed when two or more elements chemically combine in fixed proportions by weight. This means that the constituents of a compound are bonded together at a molecular level, resulting in a substance that has distinct properties differing from the individual elements that formed it. For example, water (H₂O) is a compound made up of two hydrogen atoms and one oxygen atom bonded in a specific ratio.

In contrast, a mixture contains two or more substances that are physically combined but not chemically bonded, and the proportions can vary. A solution is a specific type of mixture where one substance is dissolved in another, typically involving a liquid. An alloy is a mixture of metals or a mixture of a metal and nonmetals, which can also include varying proportions, but is not specifically defined by the fixed weight proportions characteristic of compounds. Thus, the definition of a compound aligns perfectly with the description provided in the question.