Definition of "epistle"

he may be hurried from this plan by the vehemence of thought, as in an ode, or drop it carelessly, as in an epistle or essay

1748, David Hume, An Enquiry Concerning Human Understanding, Section III, § 5

At this moment his page entered with a packet. "Lights!" said Evelyn, carelessly—for, as our readers will have already divined, he was the melancholy soliloquist—"I may as well read the Cardinal's epistle at once;—but I am no tool for his purpose.

1834, L[etitia] E[lizabeth] L[andon], chapter XXVIII, in Francesca Carrara. […], volume III, London: Richard Bentley, […], (successor to Henry Colburn), page 232

"Dear me, Mr. Holmes. Dear me!" said this singular epistle. There was neither superscription nor signature. I laughed at the quaint message; but Holmes showed unwonted seriousness.

1915, Arthur Conan Doyle, The Valley of Fear

Even last century scholars had begun to search for the cities in Asia Minor whose names have become so familiar to the Christian world through the Acts of the Apostles and the Epistles of St. Paul.

1956 — Werner Keller (translated by William Neil), The Bible as History, revised English edition, Chapter 41, page 358

Deuinitie (the Heauen of all Artes) for a while drew his thoughts vnto it, but shortly after the world the flesh and the diuell with-drewe him from that, and needes he would be of a more Gentleman-like lustie cut; whereupon hee fell to morrall Epistling and Poetrie.

1596, Thomas Nashe, “The life and godly education from his childhood of that thrice famous clarke, and morthie Orator and Poet Gabriell Haruey”, in Have with You to Saffron-Walden‎, London: John Danter

And though antient Tragedy use no Prologue, yet using sometimes, in case of self defence, or explanation, that which Martial calls an Epistle; in behalf of this Tragedy coming forth after the antient manner, much different from what among us passes for best, thus much before-hand may be Epistl’d; that Chorus is here introduc’d after the Greek manner, not antient only but modern, and still in use among the Italians.

1671, John Milton, “Of that sort of Dramatic Poem which is call’d Tragedy”, in Paradise Regain’d, to which is added Samson Agonistes‎, London: John Starkey, page 4