The Collaborative International Dictionary of English v.0.48:

Tomentum \To*men"tum\, n.; pl. Tomenta. [L. See Tomentose. ]
   (Bot.)
   The closely matted hair or downy nap covering the leaves or
   stems of some plants.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tomentum
    n 1: filamentous hairlike growth on a plant; "peach fuzz" [syn:
         hair, fuzz, tomentum]
    2: a network of tiny blood vessels between the cerebral surface
       of the pia mater and the cerebral cortex [syn: tomentum,
       tomentum cerebri]