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]

The Collaborative International Dictionary of English v.0.48:

Per diem \Per di"em\ [L.]
   By the day; substantively (chiefly U. S.), an allowance or
   amount of so much by the day. Also used adjectivally; as, a
   per diem allowance.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

per diem
    adv 1: one every day; "we'll save 100 man-hours per diem" [syn:
           per diem, by the day]
    n 1: a daily allowance for living expenses (especially while
         traveling in connection with your job)