Epistle - Vine's Expository Dictionary of New Testament Words
Epistle[ 1,,G1992, epistole ]
primarily a message" (from epistello, "to send to"), hence, "a letter, an epistle," is used in the singular, e.g., Acts 15:30; in the plural, e.g., Acts 9:2; 2 Corinthians 10:10. "Epistle is a less common word for a letter. A letter affords a writer more freedom, both in subject and expression, than does a formal treatise. A letter is usually occasional, that is, it is written in consequence of some circumstance which requires to be dealt with promptly. The style of a letter depends largely on the occasion that calls it forth." * [* From Notes on Thessalonians, by Hogg and Vine, p. 5.] "A broad line is to be drawn between the letter and the epistle. The one is essentially a spontaneous product dominated throughout by the image of the reader, his sympathies and interests, instinct also with the writer's own soul: it is virtually one half of an imaginary dialogue, the suppressed responses of the other party shaping the course of what is actually written ...; the other has a general aim, addressing all and sundry whom it may concern: it is like a public speech and looks towards publication" (J. V. Bartlet, in Hastings' Bib. Dic.)
In 2 Peter 3:16 the Apostle includes the Epistles of Paul as part of the God-breathed Scriptures.