If we are going to define what is a test plan it must be clear that it is about test objectives. It is related to a test strategy which defines the activities that meet the test objectives. So, let us consider test plan objectives.
The primary objectives of test activities should be finding defects, preventing defects, assess application quality, and meet the business or user requirements. These items are motivation for the test requirements.
Alright so what goes into a test plan? Start with what are the test requirements. Then define quality objectives, define a test methodology to determine when testing begins and completes, define a defect life cycle process, define a test management process or a test life cycle process. It is good also to define the team, roles, and responsibilities.
More detail to come.