I have not given a definitionof a chemtrail; however, I can . . .

Chemtrail: 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 or aerial firefighting;
(however, cloud seeding in some advanced forms may well be a type of Chemtrail).
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 supposedly uncharacteristic
(not always but sometimes) sky tracks.
(My working definition recognizes chemtrails may not be visible on a persistent basis just as contrails may not be visible; however, the substances in Chemtrails are not those the public is aware of and has not given consent for their use).
[link to en.wikipedia.org]

The red above are my changes to Wikipedia's definition.