What does defog mean?
"Defog" is a practical verb meaning to remove fog, mist, or condensation from a surface so that it becomes clear and usable again. It most often appears in connection with cars — drivers defog the windshield or rear window using heaters, air conditioning, or a dedicated defroster setting — but it also applies naturally to bathroom mirrors after a hot shower, eyeglasses, camera lenses, and goggles. The verb can be used transitively, as in "defog the windshield," or intransitively, as in "the mirror slowly defogged." It belongs to a productive family of verbs formed with the prefix "de-", which reverses an action or state, much like "defrost" and "de-ice." In everyday register it is neutral and slightly informal; British speakers frequently use the near-synonym "demist." A related noun, "defogger," names the device or setting that does the clearing. Though humble, the word captures one of modern life's small conveniences: restoring visibility at the push of a button.
To remove fog, mist, or condensation from a surface such as glass, a mirror, or a lens.
"He switched on the blower to defog the windshield."
Both transitive and intransitive; common in automotive contexts.
(Intransitive) To become clear as condensation on a surface dissipates.
"Wait a moment for the mirror to defog before you shave."
Your car's "defog" button exists because in 1919 a woman named Mary Anderson patented the windshield wiper — but it took decades more before drivers stopped wiping glass with their sleeves.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of defog
"Defog" was formed in English by adding the reversing prefix "de-" to "fog," a noun whose own origins are uncertain but attested by the mid-sixteenth century, possibly of Scandinavian origin. The compound verb arose in the early twentieth century alongside the spread of automobiles and enclosed glass surfaces that needed clearing. Its closest relatives are other "de-" verbs such as "defrost" and "de-ice," which emerged in the same era of mechanical convenience.
Related word forms
How defog is actually used
"Defog" is informal-to-neutral and common in everyday American usage, especially regarding cars. British English often prefers "demist" for windscreens. The word applies both transitively ("defog the glass") and intransitively ("the mirror defogged").
Easily confused with defog
"Defog" removes condensation from a surface like glass, while "defrost" removes ice or frost, typically from a freezer or frozen food.