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

The bulb is placed in the shoes with a cord connecting it to the power source. — Bestreviews, Mercury News, 3 Aug. 2026

The glow of ten-watt bulbs peeked from behind the patchworks of plywood and concrete. — Literary Hub, 3 Aug. 2026

Depending on the vehicle, this was generally as simple as popping the hood, removing a cap from the back of the headlight, and changing the bulb. — Matthew MacConnell, ArsTechnica, 3 Aug. 2026

Those bulbs are dormant now, so shop for and plant bugle lilies, species gladiolas (a million times better than gaudy, fancy hybrid gladiolus), native mariposa lilies, baboon flower, and more. — Nan Sterman, San Diego Union-Tribune, 1 Aug. 2026

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