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Nautical. any heavy material carried temporarily or permanently in a vessel to provide desired draft and stability.
Aeronautics. something heavy, as bags of sand, placed in the car of a balloon for control of altitude and, less often, of attitude, or placed in an aircraft to control the position of the center of gravity.
anything that gives mental, moral, or political stability or steadiness:
the ballast of a steady income.
gravel, broken stone, slag, etc., placed between and under the ties of a railroad to give stability, provide drainage, and distribute loads.
Electricity.

Also called ballast resistor. a device, often a resistor, that maintains the current in a circuit at a constant value by varying its resistance in order to counteract changes in voltage.
a device that maintains the current through a fluorescent or mercury lamp at the desired constant value, sometimes also providing the necessary starting voltage and current.

to furnish with ballast:
to ballast a ship.
to give steadiness to; keep steady:
parental responsibilities that ballast a person.
in ballast, Nautical. carrying only ballast; carrying no cargo.
Contemporary Examples

But are the ballast masses, so critical for the Apollo entry guidance to work properly, really gone as they should be?
Curiosity’s Mars Landing Narrated Moment by Moment by Flight Director Keith Comeaux Keith Comeaux August 6, 2012

Historical Examples

Her ballast was silver, her cargo gold and emeralds and rubies.
School Reading by Grades James Baldwin

The vessel was in ballast, and had brought money to make her purchases with.
Ned Myers James Fenimore Cooper

On vessels in ballast without passengers or cargo forty per cent.
The Panama Canal J. Saxon Mills

You are in the same boat, and we must divide the ballast a little more equally.’
Barnaby Rudge Charles Dickens

Place it upon its side and weight it with lead or other material, making this ballast fast so that it cannot move about.
Three Hundred Things a Bright Boy Can Do Anonymous

Four or five of these busts had been struck into the launch as ballast.
Homeward Bound James Fenimore Cooper

It carries an immense quantity of iron, or even lead, ballast.
The Sailor’s Word-Book William Henry Smyth

Will you take me down to the Point when you get the ballast?
Little By Little William Taylor Adams

They are like the ballast with which every ship is always loaded, at once to keep it upright and enable it to sail properly.
The Kingdom of God is Within You Count Leo Tolstoy

noun
any dense heavy material, such as lead or iron pigs, used to stabilize a vessel, esp one that is not carrying cargo
crushed rock, broken stone, etc, used for the foundation of a road or railway track
coarse aggregate of sandy gravel, used in making concrete
anything that provides stability or weight
(electronics) a device for maintaining the current in a circuit
verb (transitive)
to give stability or weight to
n.

“heavy material used to steady a ship,” 1520s, from Middle English bar “bare” (see bare; in this case “mere”) + last “a load, burden,” or borrowed from identical terms in North Sea Germanic and Scandinavian (cf. Old Danish barlast, 14c.). “Mere” because not carried for commercial purposes. Dutch balg-last “ballast,” literally “belly-load,” is a folk-etymology corruption.