4. Elet. O envoltório de vidro de uma lâmpada.
6. Quím. Parte dilatada de um tubo de vidro.http://aulete.uol.com.br/bulbo
Lightbulb would be lâmpada elétrica.
Lightbulbs could be conventional (good ole incandescent ones, the grandmother of lamps), a fluorescent lamp with that format, or even a LED lightbulb is commom these days.
Light bulb - refer to the "bulbo" itself. But sometimes they use as "eletric lamp" in some sites (beware of the context, then.)