fluid ounce

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

• noun A unit of volume or capacity in the US Customary System, used in liquid measure, equal to 29.57 milliliters (1.804 cubic inches).
• noun A unit of volume or capacity in the British Imperial System, used in liquid and dry measure, equal to 28.41 milliliters (1.734 cubic inches).

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

• noun Any of various units of volume used for liquid measure, all approximately 1.73–1.83 cubic inches or 28.4–30 milliliters.

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

• noun a United States unit of capacity or volume equal to 1.804 cubic inches
• noun a British imperial unit of capacity or volume (liquid or dry) equal to 8 fluid drams or 28.416 cubic centimeters (1.734 cubic inches)