The term 'Jet Trail' is a phrase used to describe the VAPOR trail left behind by a jet aircraft as it flies through the sky. The crossword solution for this term is 'VAPOR, ' which is a gas composed of tiny particles suspended in the air. Jet trails are created when the hot exhaust from the jet engine mixes with the cold air in the atmosphere, causing the water VAPOR in the exhaust to condense and form a visible trail.