Tear gas is a gas that makes the eyes smart and water, thus producing a temporary blindness, used in modern warfare, to quell riots, etc.

Tear gas Also called: lacrimator any one of a number of gases or vapours that makes the eyes smart and water, 
causing temporary blindness; usually dispersed from grenades and used in warfare and to control, defeat or to suppress riots 
tear gas [teer] 
noun.