Plain English Meaning
The plumbing inside the engine that keeps oil moving to all the parts that rub against each other, so they don't overheat or wear out. It also keeps the oil clean and at the right temperature.
Full Definition
The engine subsystem that stores, circulates, filters, and cools oil to lubricate moving parts, reduce friction and wear, carry away heat, and help seal the gap between pistons and cylinder walls. In most light aircraft engines, it consists of an oil sump or tank, an oil pump, an oil filter or screen, an oil cooler, oil lines, pressure and temperature sensors, and a dipstick or sight gauge for checking quantity.