The Collaborative International Dictionary of English v.0.48:

Dilatation \Dil`a*ta"tion\, n. [OE. dilatacioun, F. dilatation,
   L. dilatatio, fr. dilatare. See Dilate, and cf. 2d
   Dilation.]
   1. Prolixity; diffuse discourse. [Obs.] "What needeth greater
      dilatation?" --Chaucer.
      [1913 Webster]

   2. The act of dilating; expansion; an enlarging on al? sides;
      the state of being dilated; dilation.
      [1913 Webster]

   3. (Anat.) A dilation or enlargement of a canal or other
      organ.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dilatation
    n 1: the state of being stretched beyond normal dimensions [syn:
         dilatation, distension, distention]
    2: the act of expanding an aperture; "the dilation of the pupil
       of the eye" [syn: dilation, dilatation]