The Collaborative International Dictionary of English v.0.48:

Prorogation \Pro`ro*ga"tion\, n. [L. prorogatio: cf. F.
   prorogation.]
   1. The act of counting in duration; prolongation. [Obs.]
      --South.
      [1913 Webster]

   2. The act of proroguing; the ending of the session of
      Parliament, and postponing of its business, by the command
      of the sovereign. [Eng.]
      [1913 Webster]

   Note: After an adjournment all things continue as they were
         at the adjournment; whereas, after a prorogation, bill
         introduced and nut passed are as if they had never been
         begun at all. --Mozley & W.
         [1913 Webster]

WordNet (r) 3.0 (2006):

prorogation
    n 1: discontinuation of the meeting (of a legislative body)
         without dissolving it

Bouvier's Law Dictionary, Revised 6th Ed (1856):

PROROGATION. To put off to another time. It is generally applied to the
English parliament, and means the continuance of it from one day to another;
it differs from adjournment, which is a continuance of it from one day to
another in the same session. 1 Bl. Com. 186.
     2. In the civil law, prorogation signifies the time given to do a thing
beyond the term prefixed. Dig. 2, 14, 27, 1. See Prolongation.