(Latin: letter; message)
2. A composition in the form of a written letter.
3. Etymology: from Middle English, "letter"; from Anglo-French, from Latin epistula, epistola. "letter"; from Greek epistol?, "message, letter"; from epistellein, "to send to"; from epi-, "on, upon, above" + stellein "to send".
2. Related to anything that is written in the form of a series of letters: Messages have have been written in an epistolary style for centuries.
3. Etymology: it appeared in English four centuries after "epistle" and can be used to describe something related to or contained in an envelope by mail or messenger; as in, "epistolary greetings" or composed of letters; as in, "an epistolary story".
This term of epistolary came from the noun "epistle" which refers to "a composition written in the form of a letter to a particular person or group."
In its original sense, "epistle" referred to one of the 21 letters; such as, those from the apostle Paul which are found in the New Testament Bible.
Dating from the 13th century, epistle came into English by way of Anglo-French and Latin from the Greek noun epistol-, meaning "message" or "letter."