Angles

• 90° — Right Angle
• 0° - 90° — Acute angle
• 90° - 180° — Obtuse angle.
• 180° - 360° — Reflex angle.

Angles are also sometimes expressed in radians, with 2 * Pi radians representing an entire revolution. A radian is therefore just over 57 degrees.

Pi itself is defined as the ratio of the circumference to the diameter for a circle. See also the separate entry for 'Pi'.

http://en.wikipedia.org/wiki/Angle