Source : Webster's Revised Unabridged Dictionary (1913)

Per diem \Per di"em\ [L.]
   By the day; substantively (chiefly U. S.), an allowance or
   amount of so much by the day.



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

Source : WordNet®

per diem
     adv : one every day; "we'll save 100 man-hours per diem" [syn: {by
           the day}]