per diem(n = noun.possession) - a daily allowance for living expenses (especially while traveling in connection with your job);
is a kind of allowance

   per diem(r = adv.all) by the day - one every day; "we'll save 100 man-hours per diem"

   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.]

per diem, adv., adj. & n.
--adv. & adj. for each day.
--n. an allowance or payment for each day.

Etymology:
L

beats per minute, bits per inch, bits per second, carpe diem, charge per unit, count per minute, cycle per second, earning per share, force per unit area, gigacycle per second, kilocycle per second, kilometers per hour, kilometres per hour, megacycle per second, miles per gallon, miles per hour, million floating point operations per second, million instructions per second, per, per annum, per capita, per capita income, per centum, per se, per year, pounds per square inch, revolutions per minute, trillion floating point operations per second, words per minute