Chemtrail conspiracy theory-one definition from Wikipedia

The chemtrail conspiracy theory holds that some trails left by aircraft are actually chemical or biological agents deliberately sprayed at high altitudes for a purpose undisclosed to the general public in clandestine programs directed by government officials.^[1]

As a result, official agencies have received thousands of complaints from people who have demanded an explanation.^[1][2] The existence of chemtrails has been repeatedly denied by government agencies and scientists around the world, who say the trails are normal contrails.^[3][4]

The United States Air Force maintains that the theory is a hoax which "has been investigated and refuted by many established and accredited universities, scientific organizations, and major media publications".^[5] The British Department for Environment, Food and Rural Affairs has stated that chemtrails "are not scientifically recognised phenomena".^[3] The Canadian Leader of the Government in the House of Commons has stated that "The term 'chemtrails' is a popularised expression, and there is no scientific evidence to support their existence."^[6]
The term chemtrail is derived from "chemical trail" in the similar fashion that contrail is an abbreviation for condensation trail. It does not refer to common forms of aerial spraying such as crop dusting, cloud seeding, skywriting, or aerial firefighting.^[7] The term specifically refers to aerial trails allegedly caused by the systematic high-altitude release of chemical substances not found in ordinary contrails, resulting in the appearance of uncharacteristic sky tracks. Supporters of this theory speculate that the purpose of the chemical release may be for solar radiation management, population control,^[1] weather control,^[2] or biological warfare/chemical warfare and claim that these trails are causing respiratory illnesses and other health problems.^[8][9]