Probably strengthened from phrasso; a signet (as fencing in or protecting from misappropriation); by implication, the stamp impressed (as a mark of privacy, or genuineness), literally or figuratively -- seal.
KJV: And he received the sign of circumcision, a seal of the righteousness of the faith which he had yet being uncircumcised: that he might be the father of all them that believe, though they be not circumcised; that righteousness might be imputed unto them also:
NASB: and he received the sign of circumcision, a seal of the righteousness of the faith which he had while uncircumcised, so that he might be the father of all who believe without being circumcised, that righteousness might be credited to them,
KJV: Nevertheless the foundation of God standeth sure, having this seal, The Lord knoweth them that are his. And, Let every one that nameth the name of Christ depart from iniquity.
NASB: Nevertheless, the firm foundation of God stands, having this seal, "The Lord knows those who are His," and, "Everyone who names the name of the Lord is to abstain from wickedness."