floppy disk

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A flexible plastic disk coated with magnetic material and covered by a protective jacket, used for storing data magnetically.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun computer hardware A removable disk used for storing digital data, measuring between 2 and 8 inches diagonally and storing between 80 KB and 240 MB.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a small plastic magnetic disk enclosed in a stiff envelope with a radial slit; used to store data or programs for a microcomputer