The Collaborative International Dictionary
Printer's ream
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.
Printer's ream, twenty-one and a half quires. [Eng.] A
common practice is now to count five hundred sheets to the
ream.
--Knight.
Wiktionary
printer's ream
n. (context UK printing dated English) Twenty-one and a half quires of paper.