What is a substance composed of two or more elements combined chemically known as?

A substance composed of two or more elements that are chemically combined is known as a compound. In chemistry, a compound is formed when elements share or transfer electrons through chemical bonds, resulting in a substance with unique properties different from those of the individual elements. For example, water (H₂O) is a compound made from two hydrogen atoms and one oxygen atom chemically bonded together.

Compounds are distinct from mixtures, where two or more substances are combined physically but do not undergo a chemical change. In mixtures, the individual components can often retain their original properties and can usually be separated by physical means. Similarly, a chemical change refers to a process where the substance undergoes a transformation at the molecular level, resulting in a new substance, while a physical change involves alterations that do not change the substance's chemical identity, such as changes in state or shape.