Transmission fluid, also known as transmission oil or gearbox oil, is a type of hydraulic fluid used in motor vehicles with an automatic transmission. Its primary function is to lubricate and cool the components within the transmission, such as gears, bearings, and other moving parts, helping them operate smoothly and efficiently while reducing friction and heat generation. The fluid also helps to regulate temperature, pressure, and flow of the transmission system, ensuring proper operation and extending its lifespan.