n :  a law in chemistry: equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules - called also Avogadro's hypothesis 
 
Avo•ga•dro, Count Amedea (1776-1856), Italian physicist and chemist. Avogadro is considered one of the founders of physical chemistry. In 1811, he made his outstanding scientific contribution: his hypothesis, now accepted as a scientific law, that equal volumes of gases at the same temperature and pressure contain the same number of molecules. This hypothesis was a landmark in 19th century chemistry and is one of the basic concepts of modern chemistry.
 
    
 
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