The Collaborative International Dictionary of English v.0.48:

Attestation \At`tes*ta"tion\, n. [L. attestatio: cf. F.
   attestation.]
   The act of attesting; testimony; witness; a solemn or
   official declaration, verbal or written, in support of a
   fact; evidence. The truth appears from the attestation of
   witnesses, or of the proper officer. The subscription of a
   name to a writing as a witness, is an attestation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

attestation
    n 1: the action of bearing witness
    2: the evidence by which something is attested

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "attestation":
   admission, affidavit, affirmation, allegation, assertion,
   asseveration, attest, authentication, authority, authorization,
   averment, avouchment, avowal, backing, backing up, bearing out,
   bill of health, bolstering, buttressing, certificate,
   certificate of proficiency, certification, circumstantiation,
   compurgation, confirmation, corroboration, corroboratory evidence,
   credential, declaration, deposition, diploma, disclosure,
   documentation, evidence, fortification, instrument in proof,
   legal evidence, navicert, notarized statement, note, profession,
   proof, proving, proving out, ratification, reinforcement,
   sheepskin, statement, statement under oath, strengthening,
   substantiation, support, supporting evidence, swearing,
   sworn evidence, sworn statement, sworn testimony, testament,
   testamur, testimonial, testimonium, testimony, ticket,
   undergirding, validation, verification, visa, vise, voucher,
   vouching, warrant, warranty, witness, word



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

ATTESTATION, contracts and evidence. The act of witnessing an instrument of
writing, at the request of the party making the same, and subscribing it as
a witness. 3 P. Wms. 254 2 Ves. 454 1 Ves. & B. 362; 3 Marsh. 146; 3 Bibb.
494; 17 Pick. 373.
     2. It will be proper to consider, 1. how it is to be made 2. bow it is
proved; 3. its effects upon the witness; 4. its effect upon the parties.
     3.- 1. The attestation should be made in the case of wills, agreeably
to the direction of the statute; Com. Dig. Estates, E 1 and in the case of
deeds or other writings, at the request of the party executing the same. A
person who sees an instrument executed, but is not desired by the parties to
attest it, is not therefore an attesting witness, although he afterwards
subscribes it as such. 3 Camp. 232. See, as to the form of attestation, 2
South. R. 449.
     4.-2. The general rule is, that an attested instrument must be proved
by the attesting witness. But to this rule there are various exceptions,
namely: 1. If he reside out of the jurisdiction of the court; 22 Pick. R.
85; 2. or is dead; 3. or becomes insane; 3 Camp. 283; 4. or has an interest;
5 T. R. 371; 5. or has married the party who offers the instrument; 2 Esp.
C. 698 6. or refuses to testify 4 M. & S. 353; 7. or where the witness
swears he did not see the writing executed; 8. or becomes infamous; Str.
833; 9. or blind; 1 Ld. Raym. 734. From these numerous cases, and those to
be found in the books, it would seem that, whenever from any cause the
attesting witness cannot be had secondary evidence may be given. But the
inability to procure the witness must be absolute, and, therefore, when be
is unable to attend from sickness only, his evidence cannot be dispensed
with. 4 Taunt. 46. See 4 Halst. R. 322; Andr. 236 2 Str. 1096; 10 Ves. 174;
4 M. & S. 353 7 Taunt. 251; 6 Serg. & Rawle, 310; 1 Rep. Const.; Co. So. Ca.
310; 5 Cranch, 13; Com. Dig. tit. Testmoigne, Evidence, Addenda; 5 Com. Dig.
441; 4 Yeates, 79.
     5.-3. When the witness attests an instrument which conveys away, or
disposes of his property or rights, he is estopped from denying the effects
of such instrument; but in such case he must have been aware of its
contents, and this must be proved. 1 Esp. C. 58.
     6.-4. Proof of the attestation is evidence of the sealing and
delivery. 6 Serg. & Rawle, 311; 2 East, R. 250; 1 Bos. & Pull. 360; 7 T. R.
266. See, in general, Starkie's Ev. part 2, 332; 1 Phil. Ev. 419 to 421; 12
Wheat. 91; 2 Dall. 96; 3 Rawle's Rep. 312 1 Ves. Jr. 12; 2 Eccl. Rep. 60,
214, 289, 367 1 Bro. Civ, Law, 279, 286; Gresl. Eq. Ev. 119 Bouv. Inst. n.
3126.