What is the word for a letter, especially in formal writing?

The term "epistle" is specifically used to refer to a formal letter, often characterized by a serious tone and a structured format. This word is typically associated with communication that has a literary quality, such as the letters found in historical texts or religious documents, such as the epistles of the Apostle Paul in the New Testament. The use of "epistle" conveys an elevated form of communication, distinguishing it from more casual or informal types of letters.

While "message," "correspondence," and "note" can refer to written communication, they do not carry the same connotation of formality and literary significance as "epistle." "Message" is more general and can refer to anything communicated, while "correspondence" typically refers to the exchange of letters rather than a specific type of letter itself. "Note" often implies a brief or informal written communication, lacking the formal quality that "epistle" embodies. Thus, "epistle" is the most appropriate word for a letter in the context of formal writing.