The Collaborative International Dictionary of English v.0.48:

Ream \Ream\, n. [OE. reme, OF. rayme, F. rame (cf. Sp. resma),
   fr. Ar. rizma a bundle, especially of paper.]
   A bundle, package, or quantity of paper, usually consisting
   of twenty quires or 480 sheets.
   [1913 Webster]

   Printer's ream, twenty-one and a half quires. [Eng.] A
      common practice is now to count five hundred sheets to the
      ream. --Knight.
      [1913 Webster]