Webster's Dictionary 
(1): ( a.) Of or pertaining to penitence, or to penance; expressing penitence; of the nature of penance; as, the penitential book; penitential tears.
(2): ( n.) A book formerly used by priests hearing confessions, containing rules for the imposition of penances; - called also penitential book.
Charles Buck Theological Dictionary 
An ecclesiastical book retained among the Romanists, in which is prescribed what relates to the imposition of penance, and the reconciliation of penitents. There are various penitentials, as the Roman penitential; that of the venerable Bede; that of pope Gregory the Third, &c.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
(Codex Pmenitentialis) is an ecclesiastical book in the Romish Church which contains everything relating to the imposition of penance (q.v.) and the reconciliation of penitents (q.v.). It appoints the time and manner of penance to be regularly imposed for every sin, and forms of prayer that are to be used for the receiving of those who entered upon penance, and reconciling penitents by solemn absolution; a method chiefly introduced in the time of the degeneracy of the Church. There are various penitentials, as the Roman Penitential, and the Penitentials of Bede, and of Theodore of Tarsus, archbishop of Canterbury, etc.