tear gas

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

• noun Any of various agents that on dispersal, usually from grenades or projectiles, irritate the eyes and cause blinding tears.

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

• noun Any lachrymatory non-lethal chemical compound that causes the eyes to sting and water and/or irritates the respiratory system, mostly used for controlling crowds during riots or as self-defense.
• verb To use tear gas.

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

• noun a gas that makes the eyes fill with tears but does not damage them; used in dispersing crowds