What is Attestation Clause?

Legal Definition
In the statutory law of wills and trusts, an attestation clause is a clause that is typically appended to a will, often just below the place of the testator's signature.

In the United States, attestation clauses were introduced into probate law with the promulgation of the first version of the Model Probate Code in the 1940s. A typical attestation clause reads:

We, the undersigned testator and the undersigned witnesses, respectively, whose names are signed to the attached or foregoing instrument declare:

This attestation clause is modeled on the Model Probate Code's version. Statutes that authorize self-proved wills typically provide that a will that contains this language will be admitted to probate without affidavits from the attesting witnesses.

The validity and form of an attestation clause is usually a matter of U.S. state law, and will vary from state to state. Many states allow attestation clauses to be added as codicils to wills that were originally drafted without them.
-- Wikipedia
Legal Definition
A clause at the end of a document, in particular a will, which sets forth the legal requirements the document must satisfy, states that those requirements have been met, and is signed by one or more witnesses. An attestation clause strengthens the presumption that the requirements have been met.
Illustrative caselaw
See, e.g. Keely v. Moore, 196 U.S. 38 (1904).
See also
Legal Definition
Wills and contracts. That clause wherein the witnesses certify that the instrument has been executed before them, and the manner of the execution of the same. The usual attestation clause to a will, is in the following formula, to wit: "Signed, sealed, published and declared by the above named A B, as and for his last will and testament, in the presence of us, who have hereunto subscribed our names as the witnesses thereto, in the presence of the said testator, and of each other." That of deeds is generally in these words " Sealed and delivered in the presence of us."

2. When there is an attestation clause to a will, unsubscribed by witnesses, the presumption, though slight, is that the will is in an unfinished state; and it must be removed by some extrinsic circumstances. 2 Eccl. Rep. 60. This 'presumption is infinitely slighter, where the writer's iutention to have it regularly attested, is to be collected only from the single vord " witnesses." Id. 214. See 3 Phillim. R. 323; S. C. 1 Eng. Eccl. R. 407.
-- Bouviers Law Dictionary
Legal Definition
That clause wherein the witnesses certify that the instrument has been executed before them, and the manner of the execution of the same.
-- Black's Law Dictionary
Legal Definition
The clause in a document wherein the witnesses state the circumstances on their attestation.
-- Ballentine's Law Dictionary