Source : Webster's Revised Unabridged Dictionary (1913)

Pro \Pro\, adv.
   For, on, or in behalf of, the affirmative side; -- in
   contrast with {con}.

   {Pro and con}, for and against, on the affirmative and on the
      negative side; as, they debated the question pro and con;
      -- formerly used also as a verb.

   {Pros and cons}, the arguments or reasons on either side.