A ream is a quantity of paper. A ream is exactly 500 sheets of paper. A short ream is 480 sheets of paper and a perfect ream is 516 sheets of paper. A short ream is 20 short quires and a ream is 20 quires. A "perfect ream" (or printer's ream) is 21.5 short quires.
Dealing with tight reams with the bare hands can lead to paper cuts.
