Litmus

[lit-muh s] /ˈlɪt məs/

noun
1.
a blue coloring matter obtained from certain lichens, especially Roccella tinctoria. In alkaline solution litmus turns blue, in acid solution, red: widely used as a chemical indicator.
/ˈlɪtməs/
noun
1.
a soluble powder obtained from certain lichens. It turns red under acid conditions and blue under basic conditions and is used as an indicator
n.

“blue dye-stuff obtained from certain lichens,” early 14c., from Middle Dutch lijkmoes (Dutch lakmoes), from lac (see lac) + moes “pulp.” Another theory is that it represents Old Norse litmose, literally “lichen for dying,” from Old Norse lita “to dye, to stain,” from litr “color, dye” (see lit (n.1)) + mos “moss.” Yet another idea connects the first element to Middle Dutch leken “to drip, leak” (see leak (v.)).

Whichever was the original word, it probably was influenced by the others. The dye is obtained from certain lichens. It is naturally blue but turns red in acid and is restored to blue by alkalis. Figurative use of litmus test is first attested 1957, from scientific use of litmus-treated paper as a chemical indicator. Litmus paper with this meaning is from 1803.

litmus lit·mus (lĭt’məs)
n.
A water-soluble blue powder derived from lichens that changes to red with increasing acidity and to blue with increasing basicity.
litmus
(lĭt’məs)
A colored powder, obtained from certain lichens, that changes to red in an acid solution and to blue in an alkaline solution. Litmus is a mixture of various closely related heterocyclic organic compounds. ◇ Litmus is typically added to paper to make litmus paper, which can be used to determine whether a solution is basic or acidic by dipping a strip of the paper into the solution and seeing how the paper changes color.
litmus [(lit-muhs)]

In chemistry, a kind of paper used to tell whether a solution is an acid or a base. Acids turn blue litmus paper red; bases turn red litmus paper blue. Other testing paper or sophisticated instruments can be used to measure the pH of a solution more precisely.

Note: The term litmus is often used to refer to a general and simple test: “Your vote on this issue is a litmus test of your political philosophy.”