:heavy material (such as rocks or water) that is put on a ship to make it steady or on a balloon to control its height in the air船中保持平衡的压舱物;(热气球的)压载物 — often used figuratively常用作比喻
His wife provided the ballast he needed in times of stress. [=his wife helped to keep him steady/stable in times of stress]遇到压力时,他的妻子是他的定心丸。