The second epistle of Peter in the Bible. The second epistle of Peter in the Bible. The definition of an epistle is an especially long and formal letter, or is a poem or other writing in letter form.A very long and formal letter is an example of an epistle.he second epistle of Peter in the Bible. The second epistle of Peter in the Bible. The definition of an epistle is an especially long and formal letter, or is a poem or other writing in letter form. A very long and formal letter is an example of an epistle.