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;
is a kind of 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 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." [1913 Webster]

litmus, n. a dye obtained from lichens that is red under acid conditions and blue under alkaline conditions.

Idiom:
litmus paper a paper stained with litmus to be used as a test for acids or alkalis.

Etymology:
ME f. ONorw. litmosi f. ON litr dye + mosi moss

litmus paper, litmus test