a gas used by some police and armed forces to control crowds of people. It hurts the eyes and makes them produce tears.

to use tear gas on someone (= a gas that hurts the eyes and makes them produce tears, used in order to control crowds of people)

Security forces clubbed and tear-gassed protesters. 安全部队用棍棒和催泪瓦斯来对付抗议者。

It's hard for people who weren't there then to understand what it was like to be tear-gassed. 对于当时不在场的人来说，很难理解被催泪瓦斯袭击的感觉。