Bulb exposure method
Bulb is a camera setting that holds the shutter open for as long as the shutter release button is pressed. In some cases, the shutter release needs to be pressed once to open the shutter and once to close it, rather than remaining pushed down.
This mode allows photographers to capture longer exposures than the ones offered by the camera (usually up to 30”).
In a Nikon, this can be set in Shutter or Manual mode by adjusting the speed above 30 seconds and entering “bulb” mode.