Source : Webster's Revised Unabridged Dictionary (1913)

Per \Per\, prep. [L. Cf. {Far}, {For-}, {Pardon}, and cf. {Par},
   prep.]
   Through; by means of; through the agency of; by; for; for
   each; as, per annum; per capita, by heads, or according to
   individuals; per curiam, by the court; per se, by itself, of
   itself. Per is also sometimes used with English words.

   {Per annum}, by the year; in each successive year; annually.
      

   {Per cent}, {Per centum}, by the hundred; in the hundred; --
      used esp. of proportions of ingredients, rate or amount of
      interest, and the like; commonly used in the shortened
      form per cent.

Source : WordNet®

per annum
     adv : by the year; every year (usually with reference to a sum of
           money paid or received); "he earned $100,000 per
           annum"; "we issue six volumes per annum" [syn: {p.a.},
           {per year}, {each year}, {annually}]