The Collaborative International Dictionary of English v.0.48:

Affidavit \Af`fi*da"vit\, n. [LL. affidavit he has made oath,
   perfect tense of affidare. See Affiance, Affy.] (Law)
   A sworn statement in writing; a declaration in writing,
   signed and made upon oath before an authorized magistrate.
   --Bouvier. --Burrill.
   [1913 Webster]

   Note: It is always made ex parte, and without
         cross-examination, and in this differs from a
         deposition. It is also applied to written statements
         made on affirmation.
         [1913 Webster]

   Syn: Deposition. See Deposition.
        [1913 Webster]

WordNet (r) 3.0 (2006):

affidavit
    n 1: written declaration made under oath; a written statement
         sworn to be true before someone legally authorized to
         administer an oath