Abbreviated to B.
In lieu of shutter speed being adjusted through the camera before the photo, bulb mode will keep the shutter open for as long as the shutter button is pressed. The term comes from the days when cameras were controlled with a pneumatic “bulb” the photographer squeezed to open and shut the shutter.
Bulb mode is used when the photographer wants shutter speeds that are slower than the camera can be set to, and is often used for long exposure photographs such as for capturing fireworks, light paintings, and night sky photography.