Definitions of bulb:

• noun:   a rounded part of a cylindrical instrument (usually at one end)

  Example: "The bulb of a syringe"

• noun:   a rounded dilation or expansion in a canal or vessel or organ
• noun:   a modified bud consisting of a thickened globular underground stem serving as a reproductive structure
• noun:   electric lamp consisting of a glass bulb containing a wire filament (usually tungsten) that emits light when heated
• noun:   lower or hindmost part of the brain; continuous with spinal cord; (`bulb' is an old term for medulla oblongata)