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Definition of Litmus
Medically reviewed by Dr. Otari Nergadze, Neurosurgeon | Updated: January 2026
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.