Definitions for "Law of definite proportions"
The notion that the ratio by mass of compounds consumed in a chemical reaction is always the same.
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.
in any chemical compound, the elements are always combined in the same proportion by mass.