reindeer moss

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of several erect, grayish, branching lichens of the genus Cladonia of Arctic and boreal regions, especially C. rangiferina, constituting the chief source of food for reindeer.

from The Century Dictionary.

• noun A lichen, Cladonia rangiferina, which constitutes almost the sole winter food for the reindeer in high northern latitudes, where it is said to attain sometimes the height of one foot.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A lichen, Cladonia rangiferina, that grows in areas of alpine tundra.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an erect greyish branching lichen of Arctic and even some north temperate regions constituting the chief food for reindeer and caribou and sometimes being eaten by humans