The Collaborative International Dictionary of English v.0.48:

Decretal \De*cre"tal\, a. [L. decretalis, fr. decretum. See
   Decree.]
   Appertaining to a decree; containing a decree; as, a decretal
   epistle. --Ayliffe.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Decretal \De*cre"tal\, n. [LL. decretale, neut. of L.
   decretalis. See Decretal, a.]
   1. (R. C. Ch.) An authoritative order or decree; especially,
      a letter of the pope, determining some point or question
      in ecclesiastical law. The decretals form the second part
      of the canon law.
      [1913 Webster]

   2. (Canon Law) The collection of ecclesiastical decrees and
      decisions made, by order of Gregory IX., in 1234, by St.
      Raymond of Pennafort.
      [1913 Webster]