A brain aneurysm is a bulge or dilation in an artery in the brain. Aneurysms occur in areas where the walls of the artery are thin or weak. Aneurysms develop in approximately one in 20 people, according to the Merck Manual.

A brain aneurysm is defined as "a bulge or ballooning in a blood vessel in the brain." Aneurysms often develop in arteries at the base of the brain and are usually the result of thin and degenerating artery walls, according to MayoClinic.com.