Definition of Epistolae

In the civil law, rescripts; opinions given by the emperors in cases submitted to them for decision. Answers of the emperors to petitions. The answers of counsellors (juris-consulti), as Ulpian and others, to questions of law proposed to them, were also called “epistolae."

Opinions written out. The term originally signified the same as literae.

That's the definition of Epistolae in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.