1. (v. t.) To render authentic; to give authority to, by the proof, attestation, or formalities required by law, or sufficient to entitle to credit.

2. (v. t.) To prove authentic; to determine as real and true; as, to authenticate a portrait.

OK accept accredit affirm amen approve attest authorize autograph avouch back back up bear out bolster buttress certify circumstantiate confirm corroborate cosign countersign demonstrate document endorse fortify give permission give the go-ahead give the imprimatur give thumbs up initial justify notarize pass pass on pass upon permit probate prove ratify reinforce rubber stamp sanction say amen to seal second sign sign and seal strengthen subscribe to substantiate support sustain swear and affirm swear to test try undergird undersign underwrite uphold validate verify visa vise vouch for warrant