Definition of litmus in English:

litmus

noun

  • [mass noun] A dye obtained from certain lichens that is red under acid conditions and blue under alkaline conditions.

    • ‘In the presence of a basic solution, litmus will lose hydronium ions and turn blue.’
    • ‘During the 1660s, Robert Boyle discovered that certain plant extracts, such as litmus, can be used to distinguish acids from bases.’

Origin

Middle English: from Old Norse lit-mosi, from litr dye + mosi moss.

Pronunciation:

litmus

/ˈlɪtməs/