A ream of paper is a quantity of sheets of the same size and quality. International standards organizations define the ream as 500 identical sheets. This ream of 500 sheets (20 quires of 25 sheets) is also known as a 'long' ream, and is gradually replacing the old value of 480 sheets, now known as a 'short' ream. Reams of 472 and 516 sheets are still current, but in retail outlets paper is typically sold in reams of 500.
Certain types of specialist papers such as tissue paper, greaseproof paper, handmade paper, and blotting paper are still sold (especially in the UK) in 'short' reams of 480 sheets (20 quires of 24 sheets). However, the commercial use of the word 'ream' for quantities of paper other than 500 is now deprecated by such standards as ISO 4046. In Europe, the DIN 6730 standard for Paper and Board includes a definition of 1 ream of A4 80gsm paper=500 sheets.
The word 'ream' derives from Old French reyme, from Spanish resma, from Arabic rizmah "bundle" (of paper), from rasama, "collect into a bundle". (The Moors brought manufacture of cotton paper to Spain.) Early variant rym (late 15c.) suggests a Dutch influence. (cf. Du. riem), probably during the time of Spanish Hapsburg control of Holland.
Read more about this topic: Units Of Paper Quantity