D defs.my
Entry 1 sense Webster, 1913

Litmus

/lĭt'-məs/ · Lit·mus · IPA /ˈlɪtməs/
01 n. A dyestuff extracted from certain lichens (Roccella tinctoria, Lecanora tartarea, etc.), as a blue amorphous mass which consists of a compo…
  1. 1.
    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.(Chem.)
Phrases & compounds
Litmus paper — unsized paper saturated with blue or red litmus, -- used in testing for acids or alkalies.