Boyd’s Bible Dictionary:

(sending to). In O. T. a letter (2 Sam. 11:14; 2 Kings 5:5-6; 2 Chron. 21:12; 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. 3:2-3).