What is a test plan in software testing?

A test plan describes scope, approach, resources, schedule, and risks for testing a release or project. It typically covers what will be tested (and explicitly out of scope), types of testing (functional, regression, performance, security), environments, entry/exit criteria, and roles. Test plans align QA with product and engineering on priorities before execution starts. They differ from test cases: the plan is the strategy; cases are the executable checks. Lightweight plans work for agile teams; regulated industries often need formal, versioned documents. Update the plan when scope, timelines, or risk profile changes mid-release. A plan nobody follows is waste - keep it living and visible.