What is a Test Plan?
A test plan defines how deliverables will be verified: what gets tested at which levels (unit, integration, system, acceptance), entry and exit criteria per level, environments and data, responsibilities, and how defects route back. It converts "we'll test it" from an intention into a resourced, scheduled commitment.
The clause that saves projects is exit criteria — the pre-agreed definition of "tested enough to proceed" — decided when heads are cool, precisely so it can't be renegotiated at 11pm before go-live.
Worked example
A metro signaling project's test plan stages it: factory acceptance per subsystem (exit: zero critical defects), site integration (exit: 72-hour continuous run), trial running (exit: 4 weeks at timetable frequency, availability ≥ 99.5%). When political pressure demands an earlier opening, the plan's pre-signed exit criteria are the engineering director's shield — the system opens when it meets the numbers everyone endorsed in writing, and it does.