Ballast
Ballast turns up in 2 fields: lighting, railroad. Here is what it means to the people in each one.
Ballast in lighting
The device that starts and limits current to fluorescent and HID lamps, which would otherwise draw unlimited current.
Gas discharge lamps have negative resistance once lit, so a ballast supplies the high starting voltage and then limits current. Magnetic ballasts are heavy coils; electronic ballasts run lamps at high frequency, which ends visible flicker and raises efficacy. Ballast factor, the ratio of light from a lamp on that ballast to a reference, lets designers tune output. LED tube retrofits come in ballast-compatible and ballast-bypass versions.
"Ballast is humming and the tubes only glow at the ends, replace it or switch the fixture to ballast-bypass LED tubes."
Ballast in railroad
The bed of crushed rock under and around the ties that holds the track in line and drains water away.
Ballast spreads the load from the ties into the roadbed, locks the ties against shifting sideways and lengthwise, and lets rainwater drain quickly. Main lines use hard, angular crushed stone such as granite or trap rock, because rounded gravel rolls under load. Ballast fouled with dirt or coal dust stops draining and pumps mud, so the track has to be undercut or cleaned to recover.
"Ballast is fouled solid through that cut, every train pumps mud up between the ties, and we've got a slow order on it again."