What does "litmus" mean?

Litmus: 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. Note: Litmus is used as a dye, and being turned red by acids and restored to its blue color by alkalies, is a common indicator or test for acidity and alkalinity. Litmus paper (Chem.), unsized paper saturated with blue or red litmus, -- used in testing for acids or alkalies.