litmus paper /ˈlɪtməs-/ noun Learner's definition of LITMUS PAPER
[noncount] technical
: special paper that is used to test how much acid is in a solution ◊ Litmus paper turns red in acid solutions and blue in alkaline solutions.石蕊试纸（遇酸性溶液变红，遇碱性溶液变蓝）