bulb

noun

ˈbəlb 

plural bulbs

1

a

: a resting stage of a plant (such as the lily, onion, hyacinth, or tulip) that is usually formed underground and consists of a short stem base bearing one or more buds enclosed in overlapping membranous or fleshy leaves

b

: a fleshy structure (such as a tuber or corm) resembling a bulb in appearance

c

: a plant having or developing from a bulb

2

: a bulb-shaped part

specifically : a glass envelope enclosing the light source of an electric lamp or such an envelope together with the light source it encloses

3

: a rounded or swollen anatomical structure

4

: a camera setting that indicates that the shutter can be opened by pressing on the release and closed by ending the pressure

bulbed

ˈbəlbd 

adjective

flasks with bulbed necks

bulblike

ˈbəlb-ˌlīk 

adjective

or bulb-like

… a large underground rhizome, a bulblike growth that extends massive roots from its underside.

—Elizabeth Schneider

Illustration of bulb

• 1 hyacinth
• 2 onion
• 3 tulip
• 4 lily

Examples of bulb in a Sentence

the bulb of the thermometer

Recent Examples on the Web

This is because some types of bulbs and dimmer switches don’t play well together. —Kamron Sanders, Better Homes & Gardens, 12 July 2025

To access the bulb’s settings, open the app and tap the room where it is installed. —PC Magazine, 5 July 2025

The blade has a rectangular tip, and the handle ends in a spherical bulb. —Aspen Pflughoeft july 4, Miami Herald, 4 July 2025

This foliage is still working making food to strengthen the bulb for the next season. —Chris McKeown, The Enquirer, 2 July 2025

Word History

Etymology

Middle English, from Latin bulbus, from Greek bolbos bulbous plant

First Known Use

15th century, in the meaning defined at sense 1a

Phrases Containing bulb

• a light bulb goes off/on (in someone's head)
• bulb baster
• compact fluorescent bulb
• compact fluorescent light bulb
• dim bulb
• dry bulb temperature
• dry bulb thermometer
• incandescent light bulb
• light bulb
• olfactory bulb

Rhymes for bulb

• lightbulb

Kids Definition

bulb

noun

ˈbəlb 

1

a

: an underground resting stage of a plant (as an onion or tulip) consisting of a short stem base bearing one or more buds enclosed in overlapping leaves

b

: a fleshy structure (as a tuber or corm) resembling a bulb in appearance or function

2

: a rounded object or part shaped more or less like a bulb

a flashlight bulb

Medical Definition

bulb

noun

ˈbəlb 

1

: a rounded dilation or expansion of something cylindrical

the bulb of a thermometer

especially : a rounded or pear-shaped enlargement on a small base

the bulb of an eyedropper

2

: a rounded part: as

a

: a rounded enlargement of one end of a part see bulb of the penis, bulb of the vestibule, end bulb, hair bulb, olfactory bulb

b

: medulla oblongata

broadly : the hindbrain exclusive of the cerebellum

c

: a thick-walled muscular enlargement of the pharynx of certain nematode worms