Of or pertaining to penitence, or to penance; expressing penitence; of the nature of penance; as, the penitential book; penitential tears.

A book formerly used by priests hearing confessions, containing rules for the imposition of penances; -- called also penitential book.

of or relating to penitence or penance; "the Day of Atonement is the great penitential day of the Hebrew calendar"

showing or constituting penance; "penitential tears"; "wrote a penitential letter apologizing for her hasty words"

A penitential is a book or set of church rules concerning the Christian sacrament of penance, a "new manner of reconciliation with God"Rouche 1987 p. 528. that was first developed by Celtic monks in Ireland in the sixth century AD. When priests heard confessions, a practice originally begun by monks to use amongst themselves as a religious cleansing ritual, they compiled unofficial handbooks that dealt with the most confessed sins and a wrote down set penances for those sins.