It is an especially long and formal letter, or is a poem or other writing in letter form.
Epistle. An Epistle (greek: ejpistolhv 'epistole') is a letter. The Epistles in the Bible are such books as Romans, 1 Corinthians, 2 Corinthians, Galatians, Philippians, 1 Thessalonians, 2 Thessalonians, Philemon, 1 John, 2 John, and 3 John. The word 'epistole' occurs 24 times in the Greek New Testament.  An epistle (/ɨˈpɪsəl/ ; Greek ἐεπιστολή, epistolē, letter) is a writing directed or sent to a person or group of people, usually an elegant and formal didactic letter. The epistle genre of letter-writing was common in ancient Egypt as part of the scribal-school writing curriculum. The letters in the New Testament from Apostles to Christians are usually referred to as epistles. The second epistle of Peter in the Bible. The second epistle of Peter in the Bible. The definition of an epistle is an especially long and formal letter, or is a poem or other writing in letter form. A very long and formal letter is an example of an epistle.  Wiktionary (0.00 / 0 votes) Rate this definition: epistle (Noun). A letter, or a literary composition in the form of a letter. epistle (Noun). One of the letters included as a book of the New Testament. Origin: From epistle, from epistola, from ἐπιστολή, from ἐπιστέλλω, from ἐπί + στέλλω. Epistle. An epistle is a writing directed or sent to a person or group of people, usually an elegant and formal didactic letter. The epistle genre of letter-writing was common in ancient Egypt as part of the scribal-school writing curriculum. Full Definition of EPISTLE. 1. capitalized a: one of the letters adopted as books of the New Testament b: a liturgical lection usually from one of the New Testament Epistles. 2. a: letter; especially: a formal or elegant letter b: a composition in the form of a letter. — epis·tler \-ˈpi-sə-lər\ noun. Epistle in the Bible: any one of the letters to the early Christians that are part of the New Testament. A written communication; a term inclusive of all forms of written correspondence, personal and official, in vogue from an early antiquity. As applied to the twenty-one letters, which constitute well-nigh one-half of the New Testament, the word epistle has come to have chiefly a technical and exclusive meaning. e-pis'-'-l (epistole, a letter, epistle; from epistello, to send to): \Contents. 1. New Testament Epistles 2. Distinctive Characteristics 3. Letter-Writing in Antiquity 4. Letters in the Old Testament 5. The noun EPISTLE has 2 senses: 1. a specially long, formal letter. 2. a book of the New Testament in the form of a letter from an Apostle. Familiarity information: EPISTLE used as a noun is rare.  e·pis·tle. n. 1. A letter, especially a formal one. See Synonyms at letter. 2. A literary composition in the form of a letter. 3. Epistle Bible. a. One of the letters included as a book in the New Testament. epistle-a specially long, formal letter. letter, missive-a written message addressed to a person or organization; mailed an indignant letter to the editor. e·pis·tle. n. 1. A letter, especially a formal one. See Synonyms at letter. 2. A literary composition in the form of a letter. 3. Epistle Bible. a. One of the letters included as a book in the New Testament. 
https://carm.org/dictionary-epistle https://en.wikipedia.org/wiki/Epistle http://www.yourdictionary.com/epistle http://www.definitions.net/definition/epistle http://www.merriam-webster.com/dictionary/epistle http://www.biblestudytools.com/dictionary/epistle/ http://www.audioenglish.org/dictionary/epistle.htm http://www.thefreedictionary.com/epistle http://www.thefreedictionary.com/Epistles
definition epistle