The Collaborative International Dictionary of English v.0.48:

Penal \Pe"nal\, a. [L. poenalis, fr. poena punishment: cf. F.
   p['e]nal. See Pain.]
   Of or pertaining to punishment, to penalties, or to crimes
   and offenses; pertaining to criminal jurisprudence: as:
   (a) Enacting or threatening punishment; as, a penal statue;
       the penal code.
   (b) Incurring punishment; subject to a penalty; as, a penal
       act or offense.
   (c) Inflicted as punishment; used as a means of punishment;
       as, a penal colony or settlement. "Adamantine chains and
       penal fire." --Milton.
       [1913 Webster]

   Penal code (Law), a code of laws concerning crimes and
      offenses and their punishment.

   Penal laws, Penal statutes (Law), laws prohibiting
      certain acts, and imposing penalties for committing them.


   Penal servitude, imprisonment with hard labor, in a prison,
      in lieu of transportation. [Great Brit.]

   Penal suit, Penal action (Law), a suit for penalties.
      [1913 Webster]

WordNet (r) 3.0 (2006):

penal
    adj 1: of or relating to punishment; "penal reform"; "penal
           code"
    2: serving as or designed to impose punishment; "penal
       servitude"
    3: subject to punishment by law; "a penal offense" [syn:
       penal, punishable]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

PENAL. That which may be punished; that which inflicts a punishment.