The Collaborative International Dictionary of English v.0.48:

Tentorium \Ten*to"ri*um\, n. [L., a tent.] (Anat.)
   A fold of the dura mater which separates the cerebellum from
   the cerebrum and often incloses a process or plate of the
   skull called the bony tentorium.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tentorium
    n 1: (anatomy) a fold of dura mater that covers the cerebellum
         and supports the occipital lobes of the cerebrum