The unit circle is a circle centered at the origin, with a radius of one.

The equation of the unit circle is u2 + v2 = 1.

Note: To avoid labeling conflicts later, the unit circle is graphed in the u-v plane, rather than the x-y plane.

The unit circle provides a visual way to think about trigonometry and trigonometric functions. The unit circle concept takes any equivalence class of similar right triangles and represents the class using a single triangle with a hypotenuse of one. The triangle is oriented in the coordinate plane with the adjacent side along the x-axis, starting at the origin with angle θ (theta).