bal·last / ˈbaləst/ • n. 1. heavy material, such as gravel, sand, iron, or lead, placed low in a vessel to improve its stability. ∎  a substance of this type carried in an airship or on a hot-air balloon to stabilize it, and jettisoned when greater altitude is required. ∎ fig. something that gives stability or substance: the film is an entertaining comedy with some serious ideas thrown in for ballast.2. gravel or coarse stone used to form the bed of a railroad track or road. ∎  a mixture of coarse and fine aggregate for making concrete.3. a passive component used in an electric circuit to moderate changes in current.• v. [tr.] (usu. be ballasted) 1. give stability to (a ship) by putting a heavy substance in its bilge.2. form (the bed of a railroad line or road) with gravel or coarse stone.

ballast XVI. prob. f. LG., of uncert. orig. (perh. f. (i) bar BARE, mere or (ii) barm bosom, hull (of a ship) + last burden).