Attestation is, like assurance, a word with a broad general meaning and a technical accounting meaning.
In general, attestation means affirmation that something is true or genuine. The technical meaning derives from its use by the American Institute of Certified Public Accountants (AICPA) to define a category of engagements, many of which are assurance engagements.
An attestation engagement, according to the AICPA, comes about when a practitioner “is engaged to issue or does issue an examination, a review, or an agreed-upon procedures report on subject matter, or an assertion about the subject matter … that is the responsibility of another party”. There are exceptions: engagements carried out according to AICPA auditing, accountancy and review, and consultancy standards are not included. Neither are expert witness, advocacy, or tax preparation and advice.
In practice it is difficult to identify any principle that sets an audit apart from an attestation engagement.
Attestation engagements are sometimes understood as being those engagements where the practitioner is asked to affirm an assertion made by somebody else. However, an audit opinion is also an affirmation of an assertion: the assertion made by the preparer of the financial statements that they are (for example) true and fair.
The difference is mostly an accident of history. Audit standards were established long before most attestation standards were a twinkle in a professional body’s eye.
It is worth noting that the meanings of ‘attestation’ (as used by the AICPA) and ‘assurance’ (as used elsewhere, particularly by the International Audit and Assurance Standards Board (IAASB) overlap but are not identical. For example, agreed-upon procedures engagements are addressed by an attestation standard by the AICPA, but the IAASB standard refers to them as related services (ie, not assurance but in similar territory).
As a result the term ‘attestation’ is mostly used in the context of applying AICPA standards and has limited applicability outside the United States.