A nickname for contrail clouds left behind by planes that refers to a theory. It is believed that these contrails serve as pollution, reducing the earth surface's exposure to sunlight. This causes global cooling.

The theory though, is that there was and possibly stil is a government conspiracy, in which they are flying military planes that purposely create these chemtrails to cause global cooling (slowing down or lessening the effects of global warming).

Contrail-like cloud left by aircraft believed by "spiritual" people to contain chemicals or crystals. These chemtrails are allegedly part of a secret government program either to save earth from global warming or to kill off half of humanity.

The means by which the Illuminati sprays its proverbial load.

bullshit conspiracy theory.

Chemtrails are used to steer and propagate energies used in
ultra-secure communications, and because they quickly lose their
coherency, chemtrails that is, to become reflective layers of clouds
over a few hours time, they can also be (are) used to form what must be
called "The Broadcast Canopy", a major tool within the radiant arsenal
and in weather modification. A huge reflective sheet is formed over a
subject population, and the energies and frequencies broadcast from
antennae farms everywhere can be attuned to achieve ends that have been
researched extensively by the CIA during Projects Paperclip, MK-ULTRA,
and others...

Broadcast Canopys also are used to perpetuate things like through-wall
radar, very long distance sound bugging of premises, computer monitoring
from afar, and general invasions of privacy...