Source : Webster's Revised Unabridged Dictionary (1913)

Penitential \Pen`i*ten"tial\, a. [Cf. F. p['e]nitentiel.]
   Of or pertaining to penitence, or to penance; expressing
   penitence; of the nature of penance; as, the penitential
   book; penitential tears. ``Penitential stripes.'' --Cowper.

         Guilt that all the penitential fires of hereafter can
         not cleanse.                             --Sir W.
                                                  Scott.

Penitential \Pen`i*ten"tial\, n. (R. C. Ch.)
   A book formerly used by priests hearing confessions,
   containing rules for the imposition of penances; -- called
   also {penitential book}.

Source : WordNet®

penitential
     adj 1: of or relating to penitence or penance; "the Day of
            Atonement is the great penitential day of the Hebrew
            calendar"
     2: showing or constituting penance; "penitential tears"; "wrote
        a penitential letter apologizing for her hasty words"