The Collaborative International Dictionary of English v.0.48:

Hillock \Hill"ock\, n.
   A small hill. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hillock
    n 1: a small natural hill [syn: knoll, mound, hillock,
         hummock, hammock]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "hillock":
   anthill, barrow, brae, butte, down, drumlin, dune, fell, foothills,
   hill, hummock, knob, knoll, molehill, monticle, monticule, moor,
   mound, sand dune, swell