A very long and formal letter is an example of an epistle.
Origin of epistle
- Middle English epistel from Old French epistle from Latin epistola from Greek epistolē from epistellein to send a message to epi- epi- stellein to send stel- in Indo-European roots
From American Heritage Dictionary of the English Language, 5th Edition
- From Old French epistre, from Latin epistola, from Ancient Greek ἐπιστολή (epistolē), from ἐπιστέλλω (epistellō, “I send a message”), from ἐπί (epi, “upon”) + στέλλω (stellō, “I prepare, send”).