Avogadro's law

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The principle that equal volumes of all gases under identical conditions of pressure and temperature contain the same number of molecules.

Century Dictionary and Cyclopedia

1. n. See law.

GNU Webster's 1913

1. n. (Chem.) a fundamental conception, according to which, under similar conditions of temperature and pressure, all gases and vapors contain in the same volume the same number of ultimate molecules; -- so named after Avogadro, an Italian scientist. Sometimes called Ampère's law.

WordNet 3.0

1. n. the principle that equal volumes of all gases (given the same temperature and pressure) contain equal numbers of molecules