The Collaborative International Dictionary of English v.0.48:

Drumlin \Drum"lin\, n. [Gael. druim the ridge of a hill.]
   (Geol.)
   A hill of compact, unstratified, glacial drift or till,
   usually elongate or oval, with the larger axis parallel to
   the former local glacial motion.
   [1913 Webster]

WordNet (r) 3.0 (2006):

drumlin
    n 1: a mound of glacial drift