Merriam-Webster Collegiate Dictionary - per

 I. preposition  Etymology: Latin, through, by means of, by — more at for  Date: 14th century  1. by the means or agency of ; through ~ bearer  2. with respect to every member of a specified group ; for each  3. according to — often used with as ~ instructions as ~ usual Usage:  Per occurs most frequently in business contexts; its use outside such contexts is often criticized but is quite widespread, especially in sense 2. Its most common and natural nonbusiness uses always involve figures, usually in relation to price $150 ~ ~formance, automobiles 32 miles ~ gallon 55 miles ~ hour, or sports averages 15 points and 9 rebounds ~ game.  II. adverb  Date: 1899 for each ; apiece a bargain at $3.50 ~  III. abbreviation  1. ~iod  2. ~son