noun

1. anything heavy carried in a ship, aircraft, or vehicle to give stability or in a balloon or airship to help control altitude
2. anything giving stability and firmness to character, human relations, etc.
3. crushed rock or gravel, as that placed between and below railroad ties

Etymology: LowG < MDu < bal, bad, useless (akin to OE bealu, bale) + last, a load

transitive verb

1. to furnish with ballast; stabilize
2. to fill in (a railroad bed) with ballast