A disc of unit radius on a plane is called a unit disc. Unless otherwise specified, the unit disc on a Cartesian plane is the unit disc about the origin, the inside of the unit circle.
A closed unit disc is the set of points whose distance from the centre is less than or equal to 1. An open unit disc is the set of points whose distance from the centre is less than 1.
The area of the unit disc is π.
See also