The Collaborative International Dictionary of English v.0.48:

Pro \Pro\, prep. [L.; akin to prae before, Gr. ?, and E. for.
   See For, prep., and cf. Prior, a.]
   A Latin preposition signifying for, before, forth.
   [1913 Webster]

   Pro confesso [L.] (Law), taken as confessed. The action of
      a court of equity on that portion of the pleading in a
      particular case which the pleading on the other side does
      not deny.

   Pro rata. [L. See Prorate.] In proportion; proportion.

   Pro re nata [L.] (Law), for the existing occasion; as
      matters are.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pro re nata
    adv 1: according to need (physicians use PRN in writing
           prescriptions); "add water as needed" [syn: as needed,
           as required, pro re nata, PRN]

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

PRO RE NATA. For the occasion as it may arise.