A letter is a written message conveyed from one person to another through a medium. Something epistolary means that it is a form of letter writing. The term usually excludes written material intended to be read in its original form by large numbers of people, such as newspapers and placards, although even these may include material in the form of an "open letter". The typical form of a letter for many centuries, and the archetypal concept even today, is a sheet of paper that is sent to a correspondent through a postal system. A letter can be formal or informal, depending on its audience and purpose. Besides being a means of communication and a store of information, letter writing has played a role in the reproduction of writing as an art throughout history. Letters have been sent since antiquity and are mentioned in the Iliad. Historians Herodotus and Thucydides mention and use letters in their writings.

An epistle is a writing directed or sent to a person or group of people, usually an elegant and formal didactic letter. The epistle genre of letter-writing was common in ancient Egypt as part of the scribal-school writing curriculum. The letters in the New Testament from Apostles to Christians are usually referred to as epistles. Those traditionally attributed to Paul are known as Pauline epistles and the others as catholic epistles.