Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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