epis·tle /ɪˈpɪsəl/
  書信
  E·pis·tle n.
  1. A writing directed or sent to a person or persons; a written communication; a letter; -- applied usually to formal, didactic, or elegant letters.
     A madman's epistles are no gospels.   --Shak.
  2. Eccl. One of the letters in the New Testament which were addressed to their Christian brethren by Apostles.
  Epistle side, the right side of an altar or church to a person looking from the nave toward the chancel.
     One sees the pulpit on the epistle side.   --R. Browning.
  E·pis·tle, v. t. To write; to communicate in a letter or by writing. [Obs.]
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  epistle
       n : especially a long, formal letter