The Collaborative International Dictionary of English v.0.48:

Usnea \Us"ne*a\, 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]

WordNet (r) 3.0 (2006):

Usnea
    n 1: widely distributed lichens usually having a greyish or
         yellow pendulous freely branched thallus [syn: Usnea,
         genus Usnea]