Thayer's Greek Lexicon

 1. to set a seal upon, mark with a seal, to seal
  1) for security: from Satan
  2) since things sealed up are concealed (as the contents of a
letter), to hide, keep in silence, keep secret
  3) in order to mark a person or a thing
1c1. to set a mark upon by the impress of a seal or a stamp
1c2. angels are said to be sealed by God
  4) in order to prove, confirm, or attest a thing
1d1. to confirm authenticate, place beyond doubt
1d  1) of a written document
1d  2) to prove one's testimony to a person that he is what
he professes to be