any of various intricately branched, spongy lichens (genus Cladonia), esp. a gray lichen (C. rangiferina) eaten by grazing animals in the Arctic and, sometimes, by people
See reindeer moss in American Heritage Dictionary 4
noun
An erect, grayish, branching lichen (Cladonia rangiferina) of Arctic regions, constituting the chief source of food for reindeer and several other herbivores.