berry aneurysm

noun

ber·​ry aneurysm ˈber-ē- 

: an aneurysm of a cerebral artery that resembles a berry or sac in shape and typically occurs at an arterial junction in the circle of Willis

The culprit is a berry aneurysm, a slight (one-eighth to one-third of an inch) ballooning of the artery in the circle of Willis …

—Tony Dajer, Discover