A comprehensive review, analysis, and testing, (software and/or hardware) performed by an objective third party to confirm (i.e., verify) that the requirements are correctly defined, and to confirm (i.e., validate) that the system correctly implements the required functionality and security requirements.
Sources:
CNSSI 4009-2015
NIST SP 800-37 Rev. 2
under independent verification and validation
from
CNSSI 4009-2015