1. A blue coloring matter obtained from Roccella tinctoria and other species of lichens, the principal component of which is azolitmin; used as an indicator (reddened by acids and turned blue again by alkalies).
The definition information for litmus is provided by Stedman's. You can search our medical dictionary here.
Stedman's, part of Lippincott Williams & Wilkins, provides a comprehensive line of health-science publications for healthcare professionals and medical students.