Wikipedia
Tertianship is the final formal period of formation in the Society of Jesus. The Provincial usually invites men to begin Tertianship three to five years after finishing Formation or Graduate Studies. It is intended to be a time in which an individual steps back to critically assess his experience of living and working in the Society of Jesus and whether this is, in fact, the life to which he is being called by Christ.
Tertianship characteristically takes place either through the course of an academic year or through two consecutive summers. During this time, the Jesuit in formation, called a "tertian", will undertake an apostolic placement of teaching or service. The tertian will also undergo a thirty-day silent retreat based upon the Spiritual Exercises of St. Ignatius of Loyola.
After two years in the novitiate, the Society will usually invite a novice to make a commitment through a profession of three vows – poverty, chastity and obedience. While this profession of three vows serves to bind a man to the Society of Jesus, the Society does not make its formal commitment to an individual Jesuit until that person has been with them for more than 15 years.