barbed wire

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

• noun Twisted strands of fence wire with barbs at regular intervals.

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

• noun Twisted strands of steel wire, often coated with zinc, having barbs evenly spaced along them; used to construct agricultural and military fences.

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

• noun strong wire with barbs at regular intervals used to prevent passage