ballast

noun

• 1heavy material, such as gravel, sand, iron, or lead, placed low in a vessel to improve its stability.
• heavy material carried in an airship or on a hot-air balloon to stabilize it, and jettisoned when greater altitude is required.
• something that gives stability or substance:the film is an entertaining comedy with some serious ideas thrown in for ballast

• 2gravel or coarse stone used to form the bed of a railroad track or road.
• a mixture of coarse and fine aggregate for making concrete.

• 3a passive component used in an electric circuit to moderate changes in current.

verb

• 1give stability to (a ship) by putting a heavy substance in its bilge:the vessel has been ballasted to give the necessary floating stability

• 2form (the bed of a railroad line or road) with gravel or coarse stone.