The (open) unit disk can also be considered to be the region in the complex plane defined by 
,
where 
denotes the complex
modulus. (The closed unit disk is similarly defined as 
.
Unit Disk
See also
Five Disks Problem, Lower Half-Disk, Semicircle, Unit Circle, Unit Disk Integral, Unit Square, Upper Half-DiskExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Unit Disk." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/UnitDisk.html