The Collaborative International Dictionary of English v.0.48:

Compendium \Com*pen"di*um\, n.; pl. E. Compendiums, L.
   Compendia. [L. compendium that which is weighed, saved, or
   shortened, a short way, fr. compendere to weigh; com- +
   pendere to weigh. See Pension, and cf. Compend.]
   A brief compilation or composition, containing the principal
   heads, or general principles, of a larger work or system; an
   abridgment; an epitome; a compend; a condensed summary.
   [1913 Webster]

         A short system or compendium of a science. --I. Watts.

   Syn: See Abridgment.
        [1913 Webster]

WordNet (r) 3.0 (2006):

compendium
    n 1: a publication containing a variety of works [syn:
         collection, compendium]
    2: a concise but comprehensive summary of a larger work