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Definition of Litmus

Litmus: A pigment used as a test for acidity and alkalinity. Litmus paper turns red in acid and blue in an alkaline solution. Litmus is prepared from lichens like Roccella tinctoria. Figuratively, a litmus test is a decision in which a single factor is the determinant.

Source: MedTerms™ Medical Dictionary
Last Editorial Review: 6/14/2012

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