quantity theoryEnglishOxford New American Dictionaryquantity theory /ˈkwän(t)ədē ˌTHiərē, , ˈkwɑn(t)ədi ˌθɪəri, ˈkwän(t)ədē ˌTHirē, ˈkwɑn(t)ədi ˌθɪri/ (also the quantity theory of money) ▸ noun Economics the hypothesis that changes in prices correspond to changes in the monetary supply.