The chemtrail conspiracy theory is the erroneous belief that long-lasting condensation trails left in the sky by high-flying aircraft are actually "chemtrails" consisting of chemical or biological agents, sprayed for nefarious purposes undisclosed to the general public. Believers in this conspiracy theory say that while normal contrails dissipate relatively quickly, contrails that linger must contain additional substances. Those who subscribe to the theory speculate that the purpose of the chemical release may be solar radiation management, weather modification, psychological manipulation, human population control, biological or chemical warfare, or testing of biological or chemical agents on a population, and that the trails are causing respiratory illnesses and other health problems.

A conspiracy theory is an explanation for an event or situation that asserts the existence of a conspiracy by powerful and sinister groups, often political in motivation, when other explanations are more probable. The term generally has a negative connotation, implying that the appeal of a conspiracy theory is based in prejudice, emotional conviction, or insufficient evidence. A conspiracy theory is distinct from a conspiracy; it refers to a hypothesized conspiracy with specific characteristics, including but not limited to opposition to the mainstream consensus among those who are qualified to evaluate its accuracy, such as scientists or historians.