ballast

1. Heavy material that is placed in the hold of a ship (or in the gondola of a balloon), to provide stability. (uncountable)
2. Anything that steadies emotion or the mind. (figuratively, uncountable)
3. Coarse gravel or similar material laid to form a bed for roads or railroads, or in making concrete; track ballast. (uncountable)
4. A material, such as aggregate or precast concrete pavers, which employs its mass and the force of gravity to hold single-ply roof membranes in place. (uncountable)
5. Loose stone or big gravel used to hold rails and tracks in place. (uncountable)
6. device used for stabilizing current in an electric circuit (e.g. in a tube lamp supply circuit) (countable)
7. That which gives, or helps to maintain, uprightness, steadiness, and security. (figuratively, uncountable)

Plural ballasts