A windsock is a conical tube that resembles a large sock. It is placed on a mast and used to determine the strength and direction of wind, usually at airports. 

In radiology the windsock sign is a classic finding seen with duodenal webs on upper GI fluoroscopy in children presenting with obstruction. 

Less commonly, the windsock sign can be seen in intimo-intimal intussussception that occurs in acute aortic dissection on CT.