Validation (of data) is the act, by or on behalf of a party, of determining whether or not that data is valid to be used for some specific purpose(s) of that party. This determination can only be made after it has been determined that data has the correct structure (syntax), and conforms to other specifications that apply to its structure (verification).
Data that has been validated is considered to be sufficiently true, accurate, real or justified (i.e.: 'verified' as defined by Merriam-Webster or Lexico (Oxford dictionary)) so that it can be used for the purpose(s) that it has been validated for.
This definition generalizes the definition of 'validation' as provided by W3C VC: "The assurance that a verifiable credential or a verifiable presentation meets the needs of a verifier and other dependent stakeholders." The W3C VC specification considers validating verifiable credentials or verifiable presentations outside of its scope.
Note that validation is distinct from verification. Verification is a test on the structure of data, whereas validation tests the validity of such data for a particular (set of) purpose(s), which also has to do with meaning. Specifically, data that fails particular verification tests can still be valid for some purpose. For example, a passport that has recently expired may still be valid to identify its bearer. The converse may also be true: data that satisfies timeliness tests, e.g. hasn't expired yet, may still not be valid for specific purposes. For example, in order to apply for a Chinese (tourist) visa, the passport must not expire within 6 months of arrival in China.