The Collaborative International Dictionary of English v.0.48:

Fontanel \Fon"ta*nel`\, n. [F. fontanelle, prop., a little
   fountain, fr. fontaine fountain. See Fountain.]
   1. (Med.) An issue or artificial ulcer for the discharge of
      humors from the body.[Obs.] --Wiseman.
      [1913 Webster]

   2. (Anat.) One of the membranous intervals between the
      incompleted angles of the parietal and neighboring bones
      of a fetal or young skull; -- so called because it
      exhibits a rhythmical pulsation.
      [1913 Webster]

   Note: In the human fetus there are six fontanels, of which
         the anterior, or bregmatic, situated at the junction of
         the coronal and sagittal sutures, is much the largest,
         and remains open a considerable time after birth.
         [1913 Webster]

WordNet (r) 3.0 (2006):

fontanel
    n 1: any membranous gap between the bones of the cranium in an
         infant or fetus [syn: fontanelle, fontanel, soft
         spot]