The Collaborative International Dictionary of English v.0.48:

Lancer \Lan"cer\, n. [Cf. F. lancier.]
   1. One who lances; one who carries a lance; especially, a
      member of a mounted body of men armed with lances,
      attached to the cavalry service of some nations.
      --Wilhelm.
      [1913 Webster]

   2. A lancet. [Obs.]
      [1913 Webster]

   3. pl. (Dancing) A set of quadrilles of a certain
      arrangement. [Written also lanciers.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

lancer
    n 1: (formerly) a cavalryman armed with a lance