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

Rheinmetall’s new technology is key for the energy transition in the field of hydrogen production. —Prabhat Ranjan Mishra, Interesting Engineering, 11 Jan. 2026

For example, wet dress rehearsals during the Artemis 1 mission revealed leaks of liquid hydrogen, which required multiple rollbacks to the VAB to address. —Mike Wall, Space.com, 10 Jan. 2026

That will involve flowing in more than 700,000 gallons of liquid hydrogen and liquid oxygen, all filled during a practice countdown, and then removing the propellants while the astronauts are safely away from the pad. —Richard Tribou, The Orlando Sentinel, 10 Jan. 2026

For one thing, electric cars will now have to compete on an even playing field with vehicles powered by other types of fuel, including diesel, hydrogen, gasoline-electric hybrid, plug-in hybrid and whatever else happens to come. —Brian Moody, AJC.com, 9 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