The Collaborative International Dictionary of English v.0.48:

Prolongation \Pro`lon*ga"tion\, n. [F. prolongation.]
   [1913 Webster]
   1. The act of lengthening in space or in time; extension;
      protraction. --Bacon.
      [1913 Webster]

   2. That which forms an additional length.
      [1913 Webster]

WordNet (r) 3.0 (2006):

prolongation
    n 1: the act of prolonging something; "there was an indefinite
         prolongation of the peace talks" [syn: prolongation,
         protraction, perpetuation, lengthening]
    2: amount or degree or range to which something extends; "the
       wire has an extension of 50 feet" [syn: extension,
       lengthiness, prolongation]
    3: the consequence of being lengthened in duration [syn:
       lengthiness, prolongation, continuation, protraction]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "prolongation":
   adjournal, adjournment, coming after, consecution, consecutiveness,
   continualness, continuance, continuation, continuity, deferment,
   deferral, descent, elongation, endurance, extension,
   extension of time, following, lengthening, line, lineage,
   lingering, logical sequence, maintenance, order,
   order of succession, perpetuation, perseverance, persistence,
   posteriority, postponement, postposition, procession,
   procrastination, production, progress, progression, prorogation,
   protraction, pursuance, putting-off, repetition, rotation, run,
   sequence, series, staying power, straight course, strain, stretch,
   stretching, stringing out, subjunction, succession, successiveness,
   suffixation, sustained action, sustenance, tabling, tension,
   uninterrupted course, unremittingness, way



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

PROLONGATION. Time added to the duration of something.
     2. When the time is lengthened during which a party is to perform a
contract, the sureties of such a party are in general discharged, unless the
sureties consent to such prolongation. See Giving time.
     3. In the civil law the prolongation of time to the principal did not
discharge the surety. Dig. 2, 14, 27; Id. 12, 1, 40.