Ballast is heavy material placed in the bottom of a ship, boat, or hot air balloon to keep it stable and balanced. Ships use ballast like rocks, sand, or water-filled tanks to prevent tipping over in rough waves. Without ballast, an empty cargo ship would sit too high in the water and might capsize in a storm. Hot air balloons carry sandbags as ballast, which the pilot can drop to make the balloon rise higher.

In railroads, the crushed rock or gravel beneath train tracks is also called ballast. It spreads out the weight of passing trains, keeps the wooden ties in place, and allows rainwater to drain away so the tracks don't shift or sink into mud.

As a verb, to ballast means to add ballast to something to make it steadier.

People sometimes use ballast figuratively to describe anything that provides stability or grounding. A challenging math class might provide intellectual ballast that keeps a student focused and engaged. The idea is the same: something heavy or substantial that keeps you steady when things get turbulent.