Authenticating something is the process of verifying its validity or accuracy. COUNTERSIGN, ESTABLISH, notarize, ENSURE, JUSTIFY, ENDORSE, AFFIRM, CORROBORATE, CONFIRM, VALIDATE, VERIFY, ATTEST, PROVE, VET, CERTIFY, SEAL, and SIGN are all words that can be used to describe the action of authenticating something. COUNTERSIGN, for example, means to SIGN a document after another person has already signed it. ESTABLISH means to set up or create something. Notarize means to CERTIFY a document with an official SEAL. ENSURE means to make sure something happens. JUSTIFY means to provide evidence or a reason for something. ENDORSE means to support or approve of something. AFFIRM means to state something as true. CORROBORATE means to provide evidence to support a claim. CONFIRM means to VERIFY something. VALIDATE means to PROVE something is valid. VERIFY means to PROVE something is true. ATTEST means to testify to the truth of something. PROVE means to demonstrate the truth of something. VET means to examine something carefully. CERTIFY means to ATTEST to the truth of something. SEAL means to close or secure something. SIGN means to mark something with one's signature.