Definition:Complex Disk/Open

Definition
Let $a \in \C$ be a complex number.

Let $R>0$ be a real number.

The open (complex) disk of center $a$ and radius $R$ is the set:
 * $B(a, R) = \{z\in \C : |z-a| < R\}$

where $|\cdot|$ denotes complex modulus.

Also see

 * Definition:Closed Complex Disk