The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         Guilt that all the penitential fires of hereafter can
         not cleanse.                             --Sir W.
                                                  Scott.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

penitential
    adj 1: showing or constituting penance; "penitential tears";
           "wrote a penitential letter apologizing for her hasty
           words" [syn: penitential, penitentiary]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "penitential":
   abject, amendatory, apologetic, ascetic, atoning, balancing,
   cleansing, compensating, compensational, compensatory,
   compunctious, contrite, counteracting, counteractive,
   counterbalancing, countervailing, expiatory, humble, humbled,
   indemnificatory, lustral, lustrational, lustrative, melted,
   offsetting, penitent, penitentiary, piacular, propitiatory,
   purgative, purgatorial, purifying, reclamatory, recompensing,
   recompensive, rectifying, redeeming, redemptive, redressing,
   regretful, reparative, reparatory, repentant, repenting,
   restitutional, restitutive, restitutory, retaliatory, righting,
   satisfactional, sheepish, softened, sorry, squaring, touched