BULBING

Put simply, it's a technique of photography where the lens of the camera is left open for long enough to capture something not usually seen by the naked eye.

As far as I can tell there is no official "minimum length of time" after which a photo is officially a "bulb photo"

It is not (necessarily) a photo of a light globe or an emerging plant - although oddly enough the former is often a major contributing component in a bulb photo and the latter is often the subject of the loosely related discipline of time lapse photography (animation).

Where did the term "bulb" come from?

I don't know for sure but I can hazard a guess that it has something to do with the fact that many of the earliest cameras used an air based shutter release system.

The shutter was opened when the bulb at the end of a tube was pressed flat and stayed open until the bulb was released. This was of course a fairly inaccurate way of timing a shutter's release but was adequate for the time because longer exposures where necessary due to the limitations of the chemicals and processes used in early days.

For example, in early flash photography, it was not possible to accurately time (synchronise) the ignition of the substance exploded to create the flash, with the lens opening. Hence, subjects sat still (hopefully), the lens was opened, the flash was ignited, the lens closed and, hey presto.

It's fair to say that most photos in the early years of photography were in essence, "bulb photos".

With the advent of newer cameras, chemicals etc it was possible (and desirable) to create mechanically timed shutter releases.

I suspect the "B" setting of these cameras was retained for backwards compatibility as well as to satisfy those who wanted to take photos in low light conditions.