The Prandtl–Glauert singularity or P.G. singularity is sometimes referred to as a vapor cone, shock collar, or shock egg. The point at which a sudden drop in air pressure occurs is generally accepted as the cause of the visible condensation cloud that often surrounds an aircraft traveling at transonic speeds. It is an example of mathematicaly singularity (I don't know what that exactly means. So anyways...). Here are some of the pics of shock collar.
I found only three good pics of Shock Collar as there aren't many.