vapor

American  

[vey-per] / ˈveɪ pər /

especially British, vapour

noun

1. a visible exhalation, such as fog, mist, steam, smoke, or noxious gas, diffused through or suspended in the air.

   The vapors rising from the bogs smelled muddy.

2. Physics. a gas at a temperature below its critical temperature.

3. a substance converted into a gaseous state for technical or medicinal uses.

4. a combination of gaseous particles of a substance and air.

5. Archaic. vapors,

   1. harmful exhalations formerly supposed to be produced within the body, especially in the stomach.

   2. mental or physical illness, such as depression or hypochondria, formerly supposed to result from such exhalations, especially in women.

6. Often Facetious. the vapors. a feeling of being overwhelmed with strong emotion.

   That guy gives the press the vapors every time he announces a new project.

7. Archaic.

   1. a strange, senseless, or fantastic notion.

   2. something insubstantial or transitory.

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verb (used with object)

1. to cause to rise or pass off in, or as if in, vapor; vaporize.

2. Archaic. to affect with the vapors; depress.

verb (used without object)

1. to rise or pass off in the form of vapor.

2. to emit vapor or exhalations.

3. to talk or act grandiloquently, pompously, or boastfully; bluster.

vapor British  

/ ˈveɪpə /

noun

1. the US spelling of vapour

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vapor Scientific  

/ vā′pər /

1. The gaseous state of a substance that is normally liquid or solid at room temperature, such as water that has evaporated into the air.

2. See more at vapor pressure See also water vapor

3. A faintly visible suspension of fine particles of matter in the air, as mist, fumes, or smoke.

4. A mixture of fine droplets of a substance and air, as the fuel mixture of an internal-combustion engine.

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Usage

The words vapor and steam usually call to mind a fine mist, such as that in the jet of water droplets near the spout of a boiling teakettle or in a bathroom after a shower. Vapor and steam, however, refer to the gaseous state of a substance. The fumes that arise when volatile substances such as alcohol and gasoline evaporate, for example, are vapors. The visible stream of water droplets rushing out of a teakettle spout is not steam. As the gaseous state of water heated past its boiling point, steam is invisible. Usually, there is a space of an inch or two between the spout and the beginning of the stream of droplets. This space contains steam. The steam loses its heat to the surrounding air, then falls below the boiling point and condenses in the air as water droplets. All liquids and solids give off vapors consisting of molecules that have evaporated from the substance. In a closed system, the vapor pressure of these molecules reaches an equilibrium at which the substance evaporates from the liquid (or solid) and recondenses on it in equal amounts.

Other Word Forms

• vaporability noun
• vaporable adjective
• vaporer noun
• vaporless adjective
• vaporlike adjective

Etymology

Origin of vapor

First recorded in 1325–75; Middle English vapour, from Latin vapor “steam,” of uncertain origin; akin to vapidus vapid ( def. ) and vappa “wine that has gone flat”; perhaps cognate with Greek kapnós “smoke” ( acapnia ( def. ) )

Explanation

When something that is normally liquid — like water — becomes a visible, gas-like substance floating in the air, it's a vapor. The fog that often settles across your town in the mornings is one kind of vapor. Mist is also a vapor, as is your frozen breath when you can see it on a cold winter day. The Latin root word is vaporem, "exhalation, steam, or heat."

Example Sentences

"Enceladus itself is a key driver of this environment, releasing huge amounts of water vapor that gets ionized, loading the magnetosphere with heavy plasma that is then pulled around as the planet spins."

From Science Daily • Apr. 3, 2026

When this fuel burns inside the engine, the water droplets rapidly turn into vapor.

From Science Daily • Mar. 13, 2026

Additional eco-friendly luxury features are the “water vapor fireplaces,” built-in water filtration, and air ventilation system, which “ensure a healthy lifestyle at all times.”

From MarketWatch • Feb. 27, 2026

“MHF is simply too dangerous to use. It is a flesh-eating, low-crawling, toxic vapor cloud. Our communities will not be safe until this chemical is gone.”

From Los Angeles Times • Feb. 8, 2026

Through endless blobs of ghostly black smoke he sped, the hanging smut wafting against the smooth plexiglass nose of the ship like an evil, damp, sooty vapor against his cheeks.

From "Catch-22" by Joseph Heller

Related Words

• condensation
• fog
• gas
• moisture
• smog
• smoke
• steam