Vapour
The answer above is almost right, but it is the temperature, not the pressure that causes it.
If the air is nearly saturated (100%RH), then the dew point will be only slightly lower than the static temperature. As the air accelerates over the top of the wing both the static temperature and pressure will reduce according to Bernoulli's theorem. The air then may be below dew point and excess water will condense as visible water droplets (vapour is actually a gaseous state and invisible).
As the air decelerates behind the wing and returns to its former temperature and pressure, it will take a few seconds for the air to reabsorb the water droplets as vapour.