Water vapour that looks like a line of white smoke behind an aeroplane as it flies. The main products of hydrocarbon fuel combustion are carbon dioxide and water vapour. At high altitudes this water vapour emerges into a cold environment, and the local increase in water vapour can raise the relative humidity of the air past saturation point. Condensation is the reverse of vapourisation.
When it’s cold outside and you exhale, the water vapour in your breath condenses into lots of tiny droplets of liquid water and ice. Moist air has water vapour in it and humid air has water vapour in it, humidity is water vapour in the air.
The diseases and cancers caused by the particulates in the atmosphere, see Lifestyles and Health, have nothing to do with the water vapour trails left behind by the combustion engined aircraft.