A ballast is a device that is used to limit electricity use by an HPS lamp. Use a dimmable ballast if you want to easily be able to switch between lights during different stages of growth.

More on electricity:

• Electrical Safety Guide For Growing Marijuana Indoors
• Growing Marijuana Safely

More electrical definitions and uses:

Power consumption | Power capacity | Electricity cables | Fuse | Grounding electricity | Transformer | Relay | Watt