The Collaborative International Dictionary of English v.0.48:

Reassurance \Re`as*sur"ance\ (r[=e]`[.a]*sh[udd]r"ans), n.
   1. Assurance or confirmation renewed or repeated. --Prynne.
      [1913 Webster]

   2. (Law) Same as Reinsurance.
      [1913 Webster]

WordNet (r) 3.0 (2006):

reassurance
    n 1: the act of reassuring; restoring someone's confidence

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "reassurance":
   aid and comfort, ascertainment, assurance, certification, check,
   checking, collation, comfort, condolence, confirmation,
   consolation, determination, easement, emboldening, encouragement,
   ensuring, establishment, heartening, inspiration, inspiriting,
   inspiritment, reassurement, relief, shred of comfort, solace,
   solacement, substantiation, support, sympathy, validation,
   verification



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

REASSURANCE. When an insurer is desirous of lessening his liability, he may
procure some other insurer to insure him from loss, for the insurance he has
made this is called reassurance.