What is Attestation?

Legal Definition
Testimony or confirmation that something is true, genuine, or authentic. An attestation is frequently in writing. For example, a witness attests a will by signing it; his or her signature may confirm, inter alia, that he or she witnessed the testator sign the will.
Illustrative caselaw
See, e.g. Keely v. Moore, 196 U.S. 38 (1904).
See also
Legal Definition
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.
-- Bouviers Law Dictionary
Legal Definition
The act of witnessing an instrument in writing, at the request of the party making the same, and subscribing it as a witness. See Attest. Execution and attestation are clearly distinct formalities; the former being the act of the party, the latter of the witnesses only.
See also
-- Black's Law Dictionary
Legal Definition
The witnessing of a signature and subscription as a witness thereto. See 43 Am. St. Rep. 118.
-- Ballentine's Law Dictionary