law of definite proportions

In chemistry, the law of definite proportions states that a chemical compound always contains exactly the same proportion of elements by mass. An equivalent statement is the law of constant composition, which states that all samples of a given chemical compound have the same elemental composition. For example, oxygen makes up 8/9 of the mass of any sample of pure water, while hydrogen makes up the remaining 1/9 of the mass. Along with the law of multiple proportions, the law of definite proportions forms the basis of stoichiometry.
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Noun
1. (chemistry) law stating that every pure substance always contains the same elements combined in the same proportions by weight
(synonym) law of constant proportion
(hypernym) law, law of nature
(classification) chemistry, chemical science

When two pure substances react to form a compound, they do so in a definite proportion by mass. For example, when water is formed from the reaction between hydrogen and oxygen, the 'definite proportion' is 1 g of H for every 8 g of O.