Litmus is a special dye that's used to determine how acidic a substance is. Scientists use litmus to test liquids or gases to see if they're an acid or a base.
Litmus is the mix of dyes that's used to make litmus paper, a scientific tool that makes it easy to see if a material has an acidic pH. Made from lichen, litmus can be absorbed onto paper which turns blue to indicate an alkaline substance and red if it's acidic. Litmus comes from the Middle Dutch word lijkmoes, which combines lac, "red resinous substance" and moes, "pulp."