A disk with radius 1. The (open) unit disk can also be considered to be the region in the complex plane defined by {z: left bracketing bar z right bracketing bar <1}, where left bracketing bar z right bracketing bar denotes the complex modulus. (The closed unit disk is similarly defined as {z: left bracketing bar z right bracketing bar <=1}.

five disks problem | lower half-disk | semicircle | unit circle | unit disk integral | unit square | upper half-disk