The Collaborative International Dictionary of English v.0.48:

Diatessaron \Di`a*tes"sa*ron\, n. [L., fr. Gr. ? (sc. ?); dia`
   through + ?, gen. of ? four (sc. ?.).]
   1. (Anc. Mus.) The interval of a fourth.
      [1913 Webster]

   2. (Theol.) A continuous narrative arranged from the first
      four books of the New Testament.
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   3. An electuary compounded of four medicines.
      [1913 Webster]