Bulb (noun)

1. A rounded underground storage organ present in some plants, consisting of a short stem surrounded by fleshy scales or leaves and serving as a reproductive structure.
2. A spherical or nearly spherical underground plant stem that serves as a food reserve and propagates the plant, e.g. onion, tulip, daffodil.

Origin:

Middle english: from old french bulbe or latin bulbus, from greek bolbos "onion, bulb".

Examples:

1. He planted a bulb in the garden.
2. The bulbs of the tulip were planted in the fall.
3. The bulb of the light was replaced.
4. The bulb of the thermometer was filled with mercury.
5. The bulb of the thermometer was broken.