RAREFACTION Meaning of RAREFACTION in English [ ˌrɛ:rɪ'fakʃ(ə)n ]■ noun reduction of the density of something, especially air or a gas.OriginC17: from med. L. rarefactio(n-) , from rarefacere 'grow thin, become rare'. Concise Oxford English vocab. Сжатый оксфордский словарь английского языка. 2004