pro re nata

from 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}]

from 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]

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

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