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]

The Collaborative International Dictionary of English v.0.48:

Pro \Pro\, adv.
   For, on, or in behalf of, the affirmative side; -- in
   contrast with con.
   [1913 Webster]

   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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Pro- \Pro-\ [L. pro, or Gr. ?. See Pro.]
   A prefix signifying before, in front, forth, for, in behalf
   of, in place of, according to; as, propose, to place before;
   proceed, to go before or forward; project, to throw forward;
   prologue, part spoken before (the main piece); propel,
   prognathous; provide, to look out for; pronoun, a word
   instead of a noun; proconsul, a person acting in place of a
   consul; proportion, arrangement according to parts.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pro
    adv 1: in favor of a proposition, opinion, etc. [ant: con]
    adj 1: in favor of (an action or proposal etc.); "a pro vote"
           [ant: anti]
    n 1: an athlete who plays for pay [syn: professional, pro]
         [ant: amateur]
    2: an argument in favor of a proposal [ant: con]