What is Digest?

Legal Definition
A collection or compilation, embodying the chief matter of numerous books in one, disposed under proper heads or titles, and usually by an alphabetical arrangement, for facility in reference. As a legal term, "digest" is to be distinguished from "abridgment. The latter is a summary or epitome of the contents of a single work, in which, as a rule, the original order or sequence of parts is preserved, and in which the principal labor of the compiler is in the matter of consolidation.

A digest is wider in its scope; is made up of quotations or paraphrased passages; and has its own system of classification and arrangement. An "index" merely points out the places where particular matters may be found, without purporting to give such matters in extenso. A "treatise" or "commentary" is not a compilation, but an original composition, though it may include quotations and excerpts. A reference to the "Digest," or "Dig.," is always understood to designate the Digest (or Pandects) of the Justinian collection; that being the digest par eminence, and the authoritative compilation of the Roman law.
-- Black's Law Dictionary