Learn in the textbookValidation, Acceptance, and Migration
DefinitionValidation testing (妥当性確認テスト) checks whether the finished system meets user requirements and business needs, in other words whether it is what was actually needed.
ExplanationIts viewpoint differs from the check of whether the system was built to the design and specification (verification, 検証); it asks whether what was built is useful to the business in the first place. The name describes the viewpoint being checked rather than one particular phase of testing.
AnalogyIt is like trying on a tailored suit not to see whether it was sewn to the pattern, but whether it suits the occasions you want to wear it to.
Memory hookIn English it is validation, from the same root as valid, so remember it as checking whether the system is valid for the people who use it.