Source : Webster's Revised Unabridged Dictionary (1913)

Epistle \E*pis"tle\, v. t.
   To write; to communicate in a letter or by writing. [Obs.]
   --Milton.

Epistle \E*pis"tle\, n. [OE. epistle, epistel, AS. epistol,
   pistol, L. epistola, fr. Gr. ? anything sent by a messenger,
   message, letter, fr. ? to send to, tell by letter or message;
   'epi` upon, to + ? to dispatch, send; cf. OF. epistle,
   epistre, F. ['e]p[^i]tre. See {Stall}.]
   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.

Source : WordNet®

epistle
     n : especially a long, formal letter