1. Botany. a. A short, modified, underground stem surrounded by usually fleshy modified leaves that contain stored food for the shoot within: an onion bulb; a tulip bulb. b. A similar underground stem or root, such as a corm, rhizome, or tuber. c. A plant that grows from a bulb.

2. A rounded projection or part: the bulb of a syringe.

3. An incandescent lamp or its glass housing.

4. Anatomy. A rounded dilation or expansion of a canal, vessel, or organ.