Attestation

[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.

The act of witnessing the signing of a document and then also signing it to verify that it was properly signed by those bound by its contents.

Investopedia Says:
Ideally the person acting as the witness to the signing of a document is an independent third party. Most often people will attest for a will or power of attorney.

The act of attending the execution of a document and bearing witness to its authenticity, by signing one's name to it to affirm that it is genuine. The certification by a custodian of records that a copy of an original document is a true copy that is demonstrated by his or her signature on a certificate.

An attestation is a declaration by a witness that an instrument has been executed in his or her presence according to the formalities required by law. It is not the same as an acknowledgment, a statement by the maker of a document that verifies its authenticity.

An attestation clause is frequently found in legal documents that must be witnessed if they are to be valid, for example, a will or a deed. It states that the instrument has been completed in the manner required by law in the presence of the witness who places his or her signature in the designated space.