Compact Fluorescent Lamp (CFL)

"A compact fluorescent lamp (CFL), also known as a compact fluorescent light or energy saving light (or less commonly as a compact fluorescent tube), is a type of fluorescent lamp. Many CFLs are designed to replace an incandescent lamp and can fit into most existing light fixtures formerly used for incandescents."

Source and more info: http://en.wikipedia.org/wiki/Compact_fluorescent_lamp