Litmus test

Litmus is a water-soluble mixture of different dyes extracted from lichens. The word comes from lytmos and Old Norse litmosi meaning dye-moss. The main use of litmus is to test whether a solution is acidic or basic. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator. Litmus turns red under acidic conditions and blue under alkaline conditions.

The more modern figurative use of the term "litmus test" is a situation in which you arrive at a conclusion based on a single factor, such as an attitude, event, or fact) is decisive. A headline such as "Eurozone Elections: Litmus Test for the EU" is an example of that usage.