液压液是液压系统的工作介质，主要作用是传递，转换，控制液压能量，其它作用有抗氧化、润滑、防锈、防腐蚀、冷却、减震和冲洗等特性要求。

以适宜之粘度，依液压系统所用油泵的种类而选择适当之粘度，同时必须具有高粘度指数才不易因温度变化而影响操作，亦需具有较高之氧化稳定性，防锈性，抗泡沫性，以及分离水份能力。

使用的液压液设备的例子包括：挖掘机和反铲挖土机、液压制动器、动力转向系统、变速箱、垃圾车、飞机、飞行控制系统、升降机、以及工业机械。

Hydraulic fluid(s), also called hydraulic liquid(s), are the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water. Examples of equipment that might use hydraulic fluids include excavators and backhoes, hydraulic brakes, power steering systems, transmissions, garbage trucks, aircraft flight control systems, lifts, and industrial machinery.