Английский Этимологический словарь - ream (n.)

- 1356, from O.Fr. reyme, from Sp. resma, from Ar. rizmah "bundle," from rasama "collect into a bundle." The Moors brought manufacture of cotton paper to Spain. Early variant rym (1470s) suggests a Du. influence (cf. Du. riem), probably during the time of Spanish Hapsburg control of Holland. Properly, 20 quires or 480 sheets, usually 500 or more to allow for waste.