Boyd’s Bible Dictionary:

(sending to). In O. T. a letter (2 Sam. 11:1414And it came to pass in the morning, that David wrote a letter to Joab, and sent it by the hand of Uriah. (2 Samuel 11:14); 2 Kings 5:5-65And the king of Syria said, Go to, go, and I will send a letter unto the king of Israel. And he departed, and took with him ten talents of silver, and six thousand pieces of gold, and ten changes of raiment. 6And he brought the letter to the king of Israel, saying, Now when this letter is come unto thee, behold, I have therewith sent Naaman my servant to thee, that thou mayest recover him of his leprosy. (2 Kings 5:5‑6); 2 Chron. 21:1212And there came a writing to him from Elijah the prophet, saying, Thus saith the Lord God of David thy father, Because thou hast not walked in the ways of Jehoshaphat thy father, nor in the ways of Asa king of Judah, (2 Chronicles 21:12); Ezra 4:6-116And in the reign of Ahasuerus, in the beginning of his reign, wrote they unto him an accusation against the inhabitants of Judah and Jerusalem. 7And in the days of Artaxerxes wrote Bishlam, Mithredath, Tabeel, and the rest of their companions, unto Artaxerxes king of Persia; and the writing of the letter was written in the Syrian tongue, and interpreted in the Syrian tongue. 8Rehum the chancellor and Shimshai the scribe wrote a letter against Jerusalem to Artaxerxes the king in this sort: 9Then wrote Rehum the chancellor, and Shimshai the scribe, and the rest of their companions; the Dinaites, the Apharsathchites, the Tarpelites, the Apharsites, the Archevites, the Babylonians, the Susanchites, the Dehavites, and the Elamites, 10And the rest of the nations whom the great and noble Asnappar brought over, and set in the cities of Samaria, and the rest that are on this side the river, and at such a time. 11This is the copy of the letter that they sent unto him, even unto Artaxerxes the king; Thy servants the men on this side the river, and at such a time. (Ezra 4:6‑11)). In N. T., a formal tract containing Christian doctrine and salutary advice.

Concise Bible Dictionary:

The name given to the twenty-one “Letters”(for this is the signification of the word ἐπιστολή, and which is often thus translated) of the New Testament. Each epistle should be regarded as a letter, and be read as a whole. The word is twice used in a figurative sense. Paul said that the saints at Corinth were his “epistle” written in his heart. They were living examples of Paul’s doctrine which could be known and read of all men. The genuine power of his work was being exhibited in them. They were also manifestly the “epistle of Christ.” By means of Paul, the Spirit of the living God had written Christ upon the fleshy tables of their heart, just as surely as God’s finger had written the law on tables of stone (2 Cor. 3:2-32Ye are our epistle written in our hearts, known and read of all men: 3Forasmuch as ye are manifestly declared to be the epistle of Christ ministered by us, written not with ink, but with the Spirit of the living God; not in tables of stone, but in fleshy tables of the heart. (2 Corinthians 3:2‑3)).