Terms of reference (TOR) define the purpose and structures of a project, committee, meeting, negotiation, or any similar collection of people who have agreed to work together to accomplish a shared goal.
[1][2] Terms of reference show how the object in question will be defined, developed, and verified.
They should also provide a documented basis for making future decisions and for confirming or developing a common understanding of the scope among stakeholders.
This article describes a TOR containing detailed definitions, while a project charter has high-level requirements, assumptions, constraints and descriptions as well as a budget summary without detail, and a milestone-only schedule.
Such a consultant or adviser may be engaged via a contract with general terms of engagement that also incorporate the terms of reference that specifically describe the consultant's task.