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litmus test

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noun
  1. Chemistry. the use of litmus paper or solution to test the acidity or alkalinity of a solution.
  2. a crucial and revealing test in which there is one decisive factor.

Origin of litmus test

First recorded in 1955–60; so called from chemical tests in which litmus or litmus paper is an indicator
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for litmus test

litmus test

noun
  1. a test to establish the acidity or alkalinity of a mixture
  2. a critical indication of future success or failure
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
litmus test in Medicine

litmus test

n.
  1. A test for chemical acidity or basicity using litmus paper.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.