What is the definition or description of: vertebral artery dissection?

Same as any artery. A dissection means the blood flow doen not all flo thru the center of the artery, but leaks between the the intimal layers and splits them, gradually becomming large enough that it impedes or stops the reg flow thru the artery. Can lead 2 occlusion or rupture.
A tear in the vessel. Arteries are made up of several layers much like an onion. Occaisionally a small tear in the most inner layer can occur. The blood flows through the artery under high pressure. This jet of blood can separate the layers causing the vessel to obstruct (close off). In the vertebral artery this could cause a stroke.