The term that describes a substance formed by the combination of two or more elements in definite proportions is a compound. A compound is a pure substance that contains two or more elements chemically combined in a fixed ratio by mass. The properties of a compound are different from the properties of the individual elements that make it up. For example, water is a compound formed by the combination of hydrogen and oxygen in a ratio of 2:1 by mass. Water has properties that are different from the properties of hydrogen and oxygen.