vapour trail n (Aeronautics) a visible trail left by an aircraft flying at high altitude or through supercold air, caused by the deposition of water vapour in the engine exhaust as minute ice crystals. Also called: condensation trail or contrail Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014