Lichen
LICH'EN, noun [Latin from Gr.]
1. In botany, the name for an extensive division of cryptogamian plants, constituting a genus in the order of Algae, in the Linnean system, but now forming a distinct natural order. They appear in the form of thin flat crust, covering rocks and the bark of trees, or in foliaceous expansions, or branched like a shrub in miniature, or sometimes only as a gelatinous mass, or a powdery substance. They are called rock moss and tree moss, and some of the liverworts are of this order. They also include the Iceland moss and the reindeer moss; but they are entirely distinct from the true mosses (Musci.)
2. In surgery, a species of impetigo, appearing in the form of a red, dry, rough, and somewhat prurient spot, that gives off small furfuraceous scales.