The Collaborative International Dictionary of English v.0.48:

Crasis \Cra"sis\ (kr?"s?s), n. [LL., temperament, fr. Gr. ????,
   fr. ???? to mix.]
   1. (Med.) A mixture of constituents, as of the blood;
      constitution; temperament.
      [1913 Webster]

   2. (Gram.) A contraction of two vowels (as the final and
      initial vowels of united words) into one long vowel, or
      into a diphthong; syn[ae]resis; as, cogo for coago.
      [1913 Webster]