In classical scholarship, editio princeps (plural: editiones principes) is a term of art. It means, roughly, the first printed edition of a work that previously had existed only in manuscripts, which could be circulated only after being copied by hand.
For example, the editio princeps of Homer is that of Demetrius Chalcondyles, now thought to be from 1488. The most important texts of classical Greek and Roman authors were for the most part produced in editiones principes in the years on either side of 1500; the printing press itself was invented around 1440.
In some cases there were possibilities of partial publication, of publication first in translation (for example from Greek to Latin), and of a usage that simply equates with first edition. For a work with several strands of manuscript tradition that have diverged, such as Piers Plowman, editio princeps is a less meaningful concept.
The term has long been extended by scholars to works not part of the Ancient Greek and Latin literatures. It is also used for legal works, and other significant documents.