noun /ɪˈpɪs.l/
he may be hurried from this plan by the vehemence of thought, as in an ode, or drop it carlessly, as in an epistle or essay
Even last century scholars had begun to search for the cities in Asia Minor whose names have become so familiar to the Chistian world through the Acts of the Apostles and the Epistles of St. Paul.
Wikipedia foundation.