Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch systems in vehicles. It transfers the force from the brake pedal to the brake calipers or wheel cylinders, which then apply the brakes. Brake fluid is a high-temperature hydraulic fluid that must maintain its viscosity and boiling point over a wide temperature range to ensure proper brake performance.

There are two main types of brake fluid: DOT 3, DOT 4, and DOT 5.1 (polyethylene glycol-based) and DOT 5 (silicone-based). The type of brake fluid used depends on the manufacturer's specifications and the type of brake system. Brake fluid should be checked regularly, as it can become contaminated over time and reduce braking performance, and should be replaced according to the manufacturer's recommended service intervals.