An epistle is a letter, but more specifically, it is most often used with a series of letters in the Christian New Testament, some of which have largely been attributed to the apostle Paul, although there is considerable debate about just how many he wrote. Others are said to have come from Peter and John, among others. Very rarely used outside the context of the Bible, an epistle is generally considered to be a formal letter rather than a personal one.
The word epistle came from the old English epistol, which was in turn borrowed from the Latin word epistula or epistola. The word originated in Greek, where it meant letter or message. It also included part of the Greek verb meaning "to send."
The most common place to find the terminology is in the Bible, where the majority of the epistles were credited to Paul. In total, Paul was thought to have written 13 epistles, most of which were addressed to churches he personally had contact with during his travels, and which where chronicled in the Acts of the Apostles, the fifth book of the Christian New Testament. While Paul's authorship to eight of the epistles is generally not challenged, some have had problems attributing other epistles to him. Those epistles with questionable authorship include Ephesians, 2 Thessalonians, 1 Timothy, 2 Timothy, and Titus.
There is no set rule for when something may be termed an epistle, though the writing is considered more formalized than a personal letter. In the Bible, for example, the epistles generally include a greeting, a word of thanks upon remembrance of those being addressed, the main body often broken up into several main points, followed by a conclusion and perhaps some advice or instruction for dealing with day-to-day problems and situations. The end of the biblical epistles could include personal instructions for individuals, and some general well wishes, but that is not true in every case. The same structure, or general outline, is followed in nearly all epistles.
Likewise, an epistle does not have to be any certain length. Some are written in a poetic form, and can be very short or even long-form poetry. Others are written to convey large amounts of information, and can be quite lengthy. The length of the writing is entirely a function of the author, who must determine when he or she has been adequately addressing the key points.