From The Art and Popular Culture Encyclopedia
Prior (or prioress) is an ecclesiastical title for a superior in some religious orders. The word is derived from the Latin for "earlier" or "first". Its earlier generic usage referred to any monastic superior. In abbeys, a prior would be lower in rank than the abbey's abbot or abbess.
From Latin prior, comparative of Old Latin *pri (“before”), from Proto-Indo-European *per- (“beyond”), *pro (“before”). Parallel to English former, as comparative form from same Proto-Indo-European root, whence also fore (thence before).