What is the definition or description of: arteriovenous malformation?

Abnormal blood vesse. Is an abnormal connection and proliferation of the connection of arteries and veins.
Congenital abnormal. An avm is an abnormal collection of blood vessels secondary to maldevelopment, where veins and arteries are inappropriately connected. These can rupture and cause hemorrhage, can expand and compress nearby tissue. So, if in brain or spinal cord, could cause serious trouble.