Source : Webster's Revised Unabridged Dictionary (1913)

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.

   {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.

Source : WordNet®

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