ballast

Short definition from Wiktionary

Noun

1. 1
   Heavy material that is placed in the hold of a ship (or in the gondola of a balloon), to provide stability.
2. 2
   Anything that steadies emotion or the mind.
3. 3
   Coarse gravel or similar material laid to form a bed for roads or railroads, or in making concrete; track ballast.
4. 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.
5. 5
   device used for stabilizing current in an electric circuit (e.g. in a tube lamp supply circuit)
6. 6
   That which gives, or helps to maintain, uprightness, steadiness, and security.

Verb

1. 1
   To stabilize or load a ship with ballast.
2. 2
   To lay ballast on the bed of a railroad track.
3. 3
   To weigh down with a ballast.