pith helmet

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

• noun A lightweight hat made from the dried pith of certain tropical Asian plants or from cork.

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

• noun A lightweight helmet-shaped hat made of the pith of the sola or shola (Indian spongewood) plant covered with white cotton and faced with green cloth. Originally used by the British army in India in the 19th century, later used more widely.
• noun A hat of similar appearance, but made of different materials.

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

• noun a lightweight hat worn in tropical countries for protection from the sun