- litmus, litmus test - a coloring material (obtained from lichens) that turns red in acid solutions and blue in alkaline solutions; used as a very rough acid-base indicator
--1 is a kind of acid-base indicator
--1 is a substance of litmus paper - litmus test - a test that relies on a single indicator; "her litmus test for good breeding is whether you split infinitives"
--2 is a kind of examination, exam, test