Terms of Reference describe the purpose and structure of a project, or collection of people who have agreed to work together to accomplish a shared goal.
Terms of reference show how the scope 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.
In Liquid Law we restrict the definition of Terms of Reference to the core elements of the constitution of the group concerned, it's make-up, how members are elected and removed, and the processes and tools with which the group makes decisions.
Delegation
Terms of Reference are often used to define what is and what is not within the remit (scope) of a sub-committee - diycommitteeguide.org 
Sub-committees are set up so that a small group of management committee members (and sometimes co-opted experts) may focus in detail on a particular issue.
This allows the management committee to ensure that sufficient attention is being paid to the detail of specific issues without one topic dominating the committee agenda at every meeting.
An example of this may be a sub-committee dealing with finance and personnel, fundraising or a specific project such as a new building or a merger.
The terms of reference of a project are often referred to as the project charter.
Core Documents
- Vision, objectives, scope and deliverables (i.e. what has to be achieved)
- Stakeholders, roles and responsibilities (i.e. who will take part in it)
Project Management
Terms of Reference are sometimes considered to be the entire range of project management documentation that an organisation requires. We consider that to go outside of a more useful definition of scope - betterevaluation.org
Terms of Reference may also include:
- Resource, financial and quality plans (i.e. how it will be achieved)
- Work breakdown structure and schedule (i.e. when it will be achieved)
- Success factors/risks and restraints.