rarefy/ˈreərɪfaɪ; `rɛrəˌfaɪ/ v(pt, pp -fied) [I, Tn esp passive 尤用於被动语态](cause sth to) become thinner or less dense （使某物）变薄, 变稀, 变稀薄 rarefying gases 变得稀薄的气体.
牛津英汉双解词典