The Collaborative International Dictionary of English v.0.48:

Tumulus \Tu"mu*lus\, n.; pl. Tumuli. [L., a mound, a
   sepulchral mound, probably from tumere to swell. Cf.
   Tumid.]
   An artificial hillock, especially one raised over a grave,
   particularly over the graves of persons buried in ancient
   times; a barrow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tumulus
    n 1: (archeology) a heap of earth placed over prehistoric tombs
         [syn: burial mound, grave mound, barrow, tumulus]