It is also known as skypunch, fallstreak hole or canal cloud. It is large circular of elliptical gap that may appear in the cirrocumulus or altocumulus clouds.
These holes are formed when temperature in the cloud is less than freezing temperature but water is still not frozen. As the water is pure so there will be hardly any ice nucleation particles. When ice crystals form it sets off a chain reaction due to the Bergeron process causing the droplets around the crystals to evaporate. This creates a large circular hole in the cloud.