business case

A justification of why the project is required for the business and what the product is going to be.

It should include an outline of the Return on Investment (ROI), or a Cost/Benefit Analysis (C/BA) for the project, the project's product and performance characteristics, major project risks, and the upside opportunities.

The project's sponsor is responsible for developing the business case.

SOURCE: apm.com

A document used to justify the commitment of resources to a project.

SOURCE: welcom.com