Wellington boot

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

• noun A boot extending to the top of the knee in front but cut low in back.
• noun Chiefly British A waterproof boot of rubber or sometimes leather reaching to below the knee and worn in wet or muddy conditions.

from The Century Dictionary.

• noun A riding-boot with leg extending upward at the rear to the angle of the knee, and high enough in front to cover the knee.
• noun A similar boot, somewhat shorter, worn under the trousers, and fitting the leg closely.

from the GNU version of the Collaborative International Dictionary of English.

• A riding boot for men, the front of which came above the knee; also, a similar shorter boot worn under the trousers.

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

• noun One of a pair of Wellington boots.

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

• noun (19th century) a man's high tasseled boot