hydrogen

noun

hy·​dro·​gen ˈhī-drə-jən 

-dər-

: a nonmetallic gaseous chemical element with atomic number 1 that is the simplest and lightest of the elements and that is used especially in the processing of fossil fuels and the synthesis of ammonia

—often used before another noun

hydrogen gas

hydrogen atoms

a hydrogen vehicle

the hydrogen industry

see Chemical Elements Table see also green hydrogen compare deuterium, tritium

hydrogenous

hī-ˈdrä-jə-nəs 

adjective

Examples of hydrogen in a Sentence

Recent Examples on the Web

Last week, India signed a deal to build the world’s most powerful hydrogen locomotive, converting a diesel engine into 3,100-hp hydrogen power. —Jijo Malayil, Interesting Engineering, 26 Jan. 2026

In every variety of ice, H2O molecules interact, with weak connections called hydrogen bonds forming between one oxygen and one hydrogen atom in separate molecules. —Meghan Bartels, Scientific American, 23 Jan. 2026

Then hydrogen leaks arose during the third rehearsal, which led to NASA not only changing the way in which liquid hydrogen is loaded, but also modifying the ground umbilical plates that send power, coolant, fuel and communications to the rocket, according to Blackwell-Thompson. —Elizabeth Howell, Space.com, 23 Jan. 2026

Plug Power, a specialist in hydrogen power, witnessed a dramatic increase in its stock price on substantial trading volume, spurred by a Reddit AMA featuring the CEO. —Trefis Team, Forbes.com, 23 Jan. 2026

Word History

Etymology

French hydrogène, from hydr- + -gène -gen; from the fact that water is generated by its combustion

First Known Use

1788, in the meaning defined above

Phrases Containing hydrogen

• green hydrogen
• heavy hydrogen
• hydrogen bomb
• hydrogen bond
• hydrogen bromide
• hydrogen chloride
• hydrogen cyanide
• hydrogen fluoride
• hydrogen iodide
• hydrogen ion
• hydrogen peroxide
• hydrogen sulfide

Rhymes for hydrogen

• dihydrogen

Kids Definition

hydrogen

noun

hy·​dro·​gen ˈhī-drə-jən 

: a chemical element that is the simplest and lightest of all chemical elements and is normally found alone as a colorless odorless highly flammable gas having two atoms per molecule see element

hydrogenous

hī-ˈdräj-ə-nəs 

adjective

Etymology

from French hydrogène "hydrogen," from hydr- "water" and -gène "producer"; so called because when hydrogen gas burns, it combines with oxygen to produce water

Medical Definition

hydrogen

noun

hy·​dro·​gen ˈhī-drə-jən 

: a nonmetallic element that is the simplest and lightest of the elements and that is normally a colorless odorless highly flammable diatomic gas

—symbol H see deuterium, tritium Chemical Elements Table

hydrogenous

hī-ˈdräj-ə-nəs 

adjective