Compounds Keep a Mass Ratio
The law of constant composition states that every pure compound contains its constituent elements in a fixed mass ratio. It is also called the law of definite proportions or Proust's law.
OpenStax Chemistry: Atoms First 2e explains that Joseph-Louis Proust showed samples of a pure compound contain the same elements in the same mass proportion in Early Ideas in Atomic Theory. Britannica summarizes the law as the statement that a chemical compound has fixed element proportions by mass in
law of definite proportions.
The phrase pure compound matters. Sugar water can be made sweeter or more dilute, but pure water is still composed of hydrogen and oxygen in the same pattern. The IUPAC Gold Book uses the idea of constant composition when defining a chemical substance in chemical substance.