epistle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A letter, especially a formal one. synonym: letter.
• noun A literary composition in the form of a letter.
• noun One of the letters included as a book in the New Testament.
• noun An excerpt from one of these letters, read as part of a religious service.

from The Century Dictionary.

• noun A written communication directed or sent to a person at a distance; a letter; a letter missive: used particularly in dignified discourse or in speaking of ancient writings: as, the epistles of Paul, of Pliny, or of Cicero.
• noun [capitalized] In liturgics, one of the eucharistic lessons, taken, with some exceptions, from an epistolary book of the New Testament and read before the gospel.
• noun Any kind of harangue or discourse; a communication.
• To write as a letter; communicate by writing or by an epistle.

from the GNU version of the Collaborative International Dictionary of English.

• transitive verb obsolete To write; to communicate in a letter or by writing.
• noun A writing directed or sent to a person or persons; a written communication; a letter; -- applied usually to formal, didactic, or elegant letters.
• noun (Eccl.) One of the letters in the New Testament which were addressed to their Christian brethren by Apostles.
• noun the right side of an altar or church to a person looking from the nave toward the chancel.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A letter, or a literary composition in the form of a letter.
• noun Christianity One of the letters included as a book of the New Testament.
• verb obsolete To write; to communicate in a letter or by writing.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a book of the New Testament written in the form of a letter from an Apostle
• noun a specially long, formal letter