"Chemtrails" (chemical + contrails) are persistent condensation trails (contrails) of ice crystals produced by high-altitude aircraft. The water vapor in the engine exhaust cools and condenses into the same type of ice crystals that form ordinary high-level clouds; and if the air at flight level is moist enough, the resulting linear cloud can persist for long periods of time.

The word "chemtrail" is used by some conspiracy theorists, who see them as evidence of government-controlled weather modification and/or biochemical warfare testing. However, the vast majority of contrails are from civilian aircraft, there is no credible evidence of atmospheric contamination associated with persistent contrails, and none of the physical features that supposedly set chemtrails apart from contrails are inconsistent with the behavior of ice-crystal clouds.