Chemtrail

The chemtrail conspiracy theory posits that some trails left by aircraft are chemical orbiological agents deliberately sprayed in the sky for purposes undisclosed to the general public and directed by various government officials.This theory has been refuted by the scientific community: such trails are simply normal contrails (condensation trails). The term chemtrail is a portmanteau of the words "chemical" and "trail", just as contrail is a contraction of "condensation trail". The term does not refer to other forms of aerial spraying such as agricultural spraying ('crop dusting'), cloud seeding, skywriting, or aerial firefighting—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 characteristic sky tracks.

Supporters of this conspiracy theory speculate that the purpose of the chemical release may be for solar radiation management, psychological manipulation, human population control,weather modification, or biological or chemical warfare, and that these trails are causing respiratory illnesses and other health problems.

Reference
Wiki [Chemtrails]