制动液也叫刹车液或者刹车油，是汽车制动系统的液压介质。制动液分为含硅制动液，和不含硅制动液。不含硅制动液的化学成分是醇类，如 glycol-ether 即乙二醇醚。

制动液的主要指标是沸点。因汽车根据帕斯卡原理传递液压。一旦制动液沸腾，则产生气阻，导致制动失效，危及安全。目前国际通用的制动液标准是DOT 美国运输部标准。DOT3是早期标准，而现在大部分汽车使用DOT4。 DOT5.1为赛车标准，沸点最高。

Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used to transfer force into pressure, and to amplify braking force. It works because liquids are not appreciably compressible — in their natural state the component molecules do not have internal voids and the molecules pack together well, so bulk forces are directly transferred to compress the fluid's chemical bonds.