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.

1495–1505; earlier lytmos < Old Norse litmosi dye-moss, equivalent to lit- color, dye + mosi moss Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
litmus (ˈlɪtməs)
a soluble powder obtained from certain lichens. It turns red under acid conditions and blue under basic conditions and is used as an indicator
[C16: perhaps from Scandinavian; compare Old Norse litmosi, from litr dye + mosi moss]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

early 14c., from M.Du. lijkmoes (Du. lakmoes), from lac (see lac) + moes "pulp." The other theory is that it represents O.N. litmose, lit. "lichen for dying," from O.N. lita "to dye, to stain," from litr "color, dye," from P.Gmc. *wlitiz (cf. O.E. wlite "brightness, beauty,"
O.Fris. wlite "exterior, form," Goth. *wlits "face, form") + mos "moss." Yet another idea connects the first element to M.Du. leken "to drip, leak." 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.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

litmus lit·mus (lĭt'məs)
A water-soluble blue powder derived from lichens that changes to red with increasing acidity and to blue with increasing basicity.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
litmus   (lĭt'məs)  Pronunciation Key 
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.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary
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.”
The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
The litmus test is whether you can spot them in advance.
Magnum's output will be observed closely and considered by many as a litmus
  paper for the industry's integrity.
Her problem seems to be that she has violated the university's litmus test for
  what it means to be open-minded.
Strategists and donors for both parties should never forget this, and should
  always choose leadership abilities over litmus tests.
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