Artery
An aneurysm is a bulge in the wall of an artery.  It is like a little balloon, filled with blood, coming off the artery.  As the aneurysm gets bigger, it is more likely to break.  If the aneurysm breaks ("ruptures"), the artery will start bleeding.  If the artery is big, a person could bleed to death.  Also, if an artery is bleeding, blood cannot get through it.  The part of the body that the artery is supposed to feed will not get the blood and oxygen it needs.  A ruptured aneurysm is a medical emergency.