The Collaborative International Dictionary of English v.0.48:

Diastole \Di*as"to*le\, n. [L., fr. Gr. ?, fr. ? to put asunder,
   to separate; dia` through + ? to set, to place.]
   1. (Physiol.) The rhythmical expansion or dilatation of the
      heart and arteries; -- correlative to systole, or
      contraction.
      [1913 Webster]

   2. (Gram.) A figure by which a syllable naturally short is
      made long.
      [1913 Webster]

WordNet (r) 3.0 (2006):

diastole
    n 1: the widening of the chambers of the heart between two
         contractions when the chambers fill with blood