Uses
The English word epistle or its plural is found 21 times in 20 King James New Testament verses. It occurs the most in 2Corinthians (5 times) followed by 2Thessalonians (4).
The first Biblical reference to an epistle is in Acts 15 where the decision that Gentile converts to Christianity did not need to be circumcised was composed in a letter and sent to Antioch.
So when they were dismissed, they (Paul, Barnabas and others) came to Antioch: and when they had gathered the multitude together, they delivered the epistle . . . (Acts 15:30, KJV throughout).
The last use of the word was in Apostle Peter's comment concerning the difficulty in understanding some of what Paul wrote.
And account that the longsuffering of our Lord is salvation; even as our beloved brother Paul also according to the wisdom given unto him hath written unto you;
As also in all his epistles, speaking in them of these things; in which are some things hard to be understood . . . (2Peter 3:15 - 16).
Definition
Epistle comes from the Greek word epistole (Strong's Concordance #G1992) which means a written message like a letter.
New Testament epistles are considered those writings addressed to either a church (or collection thereof) or to one or more people. Written to tackle specific needs and issues, they repeatedly conveyed Christian truths applicable to different situations.
Twenty-one out of twenty-seven books, or about 78% of the New Testament, are considered epistles. The only six writings not considered in this set are the four gospels, the book of Acts and Revelation.
A Perfect Set of Seven
The General Epistles are a collection of seven New Testament books written by James (1), Peter (2), John (3) and Jude (1). They are referenced as "general" as they are not addressed to a particular church or person but rather to all Christians (or those in several locations).
In most Bible translations this set of books is placed between Hebrews and Revelation. In the Bible's originally inspired order, however, they make up the fifth division of Scripture. They come after the fourth division (all four gospel accounts plus Acts) but before the sixth division (all of Paul's epistles).
Paul's Writings
Paul's fourteen writings, in almost all Biblical translations, are placed after the book of Acts but before the book of James. The Bible's original order, however, finalized by the Apostle John (Holy Bible, a Faithful Version, 2nd. Ed., pg. 72), places them after the book of Jude (the last of the General Epistles) but before Revelation. They form the sixth inspired division of the Bible.
Pauline Subsets
The Apostle Paul's epistles can further be subdivided into three subsets. The first set are those books written to churches in Rome, Corinth, the province of Galatia (Pisidian Antioch, Iconium, Lystra and Derbe), Ephesus, Philippi, Colosse and Thessalonica.
The second set of Paul's work is comprised of Hebrews which was written for the entire church. The third and final set, known as Paul's pastoral epistles, was written to individuals (1 and 2 Timothy, Titus and Philemon). This last set includes instructions to Christian leaders (those who worked with Paul) about how best to serve, requirements for those seeking leadership responsibilites and so on.
Hidden References!
Although the book of Acts is not considered an epistle, it does mention or record some important letters to groups or individuals within it!
The first important letter is the one Saul (later Paul) received from the High Priest giving him permission to hunt for and arrest believers found in the synagogues of Damascus (Acts 9:1 - 2).
The second letter in Acts, which is labeled an epistle (Acts 15:30), is the one sent by the Jerusalem Conference giving their decision on the circumcision of Gentile Christians (verses 23 - 29).
The third letter in Acts, also called an epistle, is Claudius Lysias' correspondence to Felix in Caesarea concerning the Apostle Paul's arrest.
Who, when they came to Caesarea, and delivered the epistle to the governor, presented Paul also before him (Acts 23:33, see also verses 25 to 30).
Conclusion
The New Testament epistles were written by only five individuals. They are James, Peter, John, Jude and Paul. The uniqueness of these writings is summed up in the following quote.
"The Epistles consequently have a charm, a directness, a vitality and power unknown to the other sacred writings of the world. Nowhere are they equaled or surpassed except in the personal instructions that fell from the lips of Jesus . . . No writings in history so unfold the nature and processes of the redemptive experience." (International Standard Bible Encyclopedia).