‖n. [ NL., from Ar. usnah moss. ] (Bot.) A genus of lichens, most of the species of which have long, gray, pendulous, and finely branched fronds. Usnea barbata is the common bearded lichen which grows on branches of trees in northern forests. [ 1913 Webster ]

a. (Chem.) Pertaining to, or designating, a complex acid obtained, as a yellow crystalline substance, from certain genera of lichens (Usnea, Parmelia, etc.). [ 1913 Webster ]