The term "old bulb" can refer to several different things depending on the context. Here are a few possibilities:

1. Incandescent Light Bulb: The traditional incandescent light bulb, invented by Thomas Edison in the late 19th century, is often referred to as the "old bulb." It works by heating a filament inside a glass bulb until it glows, producing light. These bulbs are less energy-efficient compared to modern alternatives like LED and CFL bulbs.

2. Vacuum Tube: In electronics, an "old bulb" might refer to vacuum tubes, which were used in early electronic devices such as radios, televisions, and computers before the advent of transistors. These glass tubes contain electrodes and are used to control electric current.

3. Historical Artifacts: In a more general sense, "old bulb" could refer to any historical or antique light bulb, such as those found in museums or collections. These might include early versions of electric lights or unique designs from different eras.

4. Figurative Use: The term could also be used figuratively to describe something outdated or obsolete, much like how "old bulb" might be used to describe an old-fashioned idea or technology.

If you have a specific context in mind, please provide more details so I can give a more precise answer.