The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

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


   per cent, Per centum, percent, by the hundred; in the
      hundred; a proportion multiplied by one hundred; -- used
      esp. of proportions of ingredients, rate or amount of
      interest, and the like; most commonly used in the
      shortened form per cent; as, 5 is ten per cent of 50. It
      is commonly symbolized with the per cent sign, "%".

   Per diem, by the day. [For other phrases from the Latin,
      see Quotations, Phrases, etc., from Foreign Languages, in
      the Supplement.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

per annum
    adv 1: 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: per
           annum, p.a., per year, each year, annually]