What is Epistle?

Alternative Forms: Epistles
Legal Definition
An epistle (/ˈpɪsəl/; Greek ἐπιστολή, epistolē, "letter") 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 (i.e., "general") epistles.
-- Wikipedia
Legal Definition
Civil law. The name given to a species of rescript. Epistles were the answers given by the prince, when magistrates submitted to him a question of law. Vicle Rescripts.
-- Bouviers Law Dictionary