- BALLAST (noun) [1.] Nautical. any heavy material carried temporarily or permanently in a vessel to provide desired draft and stability. [2.] Aeronautics. something heavy, as bags of sand, placed in the car of a balloon for control of altitude and, less often, of attitude, or placed in an aircraft to control the position of the center of gravity. [3.] anything that gives mental, moral, or political stability or steadiness: the ballast of a steady income. [4.] gravel, broken stone, slag, etc., placed between and under the ties of a railroad to give stability, provide drainage, and distribute loads. [5.] Electricity. (a.)Also called ballast resistor. a device, often a resistor, that maintains the current in a circuit at a constant value by varying its resistance in order to counteract changes in voltage. (b.) a device that maintains the current through a fluorescent or mercury lamp at the desired constant value, sometimes also providing the necessary starting voltage and current.