Emily Dickinson Lexicon

LICH'EN, n. [L. from Gr. κειχην.]

1. In botany, the name for an extensive division of cryptogamian plants, constituting a genus in the order of Algæ in the Linnean system, but now forming a distinct natural order. They appear in the form of thin flat crusts, 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.) – Ed. Encyc.
2. In medicine, a papular cutaneous eruption consisting of diffuse red pimples, which are attended with a trouble some sense of tingling and pricking. A common variety of this affection resembles the effect of stinging with nettles, and is called nettle-lichen.