Confirmation (through the provision of strong, sound, objective evidence) that requirements for a specific intended use or application have been fulfilled (e.g., a trustworthy credential has been presented, or data or information has been formatted in accordance with a defined set of rules, or a specific process has demonstrated that an entity under consideration meets, in all respects, its defined attributes or requirements).
Sources:
CNSSI 4009-2015
NIST SP 800-12 Rev. 1 under Validation from CNSSI 4009

  Confirmation, through the provision of objective evidence, that the requirements for a specific intended use or application have been fulfilled.
Sources:
NIST SP 800-160v1r1 from ISO 9000:2015
NIST SP 800-161r1 from ISO 9000:2015

  The process of determining that an object or process is acceptable according to a pre-defined set of tests and the results of those tests.
Sources:
NIST SP 1800-16B under Validation from NIST SP 800-152
NIST SP 1800-16C under Validation from NIST SP 800-152
NIST SP 1800-16D under Validation from NIST SP 800-152
NIST SP 800-152 under Validation

  Confirmation (through the provision of strong, sound, objective evidence) that requirements for a specific intended use or application have been fulfilled.
Sources:
NISTIR 7622 under Validation from ISO 9000

  Confirmation (through the provision of strong, sound, and objective evidence and demonstration) that requirements for a specific intended use or application have been fulfilled and that the system, while in use, fulfills its mission or business objectives while being able to provide adequate protection for stakeholder and mission or business assets, minimize or contain asset loss and associated consequences, and achieve its intended use in its intended operational environment with the desired level of trustworthiness.
Sources:
NIST IR 8323r1
NIST IR 8401 from NIST SP 800-160v1r1 - adapted