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

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "affidavit":
   admission, affirmation, allegation, assertion, asseveration,
   attest, attestation, authority, authorization, averment,
   avouchment, avowal, bill, bill of complaint, bill of health,
   certificate, certificate of proficiency, certification, claim,
   complaint, compurgation, credential, declaration, deposition,
   diploma, disclosure, instrument in proof, legal evidence, libel,
   manifesto, narratio, navicert, nolle prosequi, nonsuit,
   notarized statement, note, position paper, profession, sheepskin,
   solemn declaration, statement, statement of belief,
   statement of facts, statement under oath, swearing, sworn evidence,
   sworn statement, sworn testimony, testamur, testimonial,
   testimonium, testimony, ticket, visa, vise, voucher, vouching,
   warrant, warranty, witness, word



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

AFFIDAVIT, practice. An oath or affirmation reduced to writing, sworn or
affirmed to before some officer who has authority to administer it. It
differs from a deposition in this, that in the latter the opposite party has
had an opportunity to cross-examine the witness, whereas an affidavit is
always taken ex parte. Gresl. Eq. Ev. 413. Vide Harr. Dig. h.t.
     2. Affidavit to hold to bail, is in many cases required before the
defendant can be arrested; such affidavit must be made by a person who is
acquainted with the fact, and must state, 1st, an indebtedness from the
defendant to the plaintiff; 2dly, show a distinct cause of action; 3dly, the
whole must be clearly and certainly, expressed. Sell. Pr. 104; 1 Chit. R.
165; S. C. 18 Com. Law, R. 59 note; Id. 99.
     3. An affidavit of defence, is made by a defendant or a person knowing
the facts, in which must be stated a positive ground of defence on the
merits. 1 Ashm. R. 4, 19, n. It has been decided that when a writ of summons
has been served upon three defendants, and only one appears, a judgment for
want of an affidavit of defence may be rendered against au. 8 Watts, R. 367.
Vide Bac. Ab. h.t.