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 stars of the show are the thousands of bulbs that pulse to music and a clown that sings along to songs. — Patrick Connolly, The Orlando Sentinel, 22 Oct. 2026

The first jerseys of the local soccer team featured a light-bulb emblem, and graffiti light bulbs are still spray-painted on the city’s underpass walls. — Gideon Lewis-Kraus, New Yorker, 19 Oct. 2026

The remarkable human brain that conceived heart surgery and interplanetary travel runs on roughly the same amount of energy as a refrigerator bulb, thanks to its trillions of synapses. — New Atlas, 18 Oct. 2026

These bulbs were phased out by the Department of Energy in favor of more efficient LED bulbs. — Karen Brewer Grossman, Southern Living, 17 Oct. 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