In kaplan textbook, assurance engagement is defined as an engagement in which a practitioner expresses a conclusion designed to enhance the confidence of the intended users.
In this definition, this practitioner refers to an accountant or auditor ?
An audit is just one type/example of an assurance engagement. Practitioner is a generic term for someone who is “in practice”. In the accounting profession, practitioners are called “professional accountants” if they are members of a professional body. A professional accountant may be either “in public practice” or “in business”. A “professional accountant in public practice” is in a firm that provides provides professional services and will be called an “auditor” if the professional service provided is audit. So, in your definition, the practitioner is an auditor.