litmus(n = noun.substance)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;
A dyestuff extracted from certain lichens (Roccella tinctoria, Lecanora tartarea, etc.), as a blue amorphous mass which consists of a compound of the alkaline carbonates with certain coloring matters related to orcin and orcein. [1913 Webster]
Litmus paper(Chem.), unsized paper saturated with blue or red litmus, -- used in testing for acids or alkalies.