The "Unit Circle" is a circle with a radius of 1.

Because the radius is 1, we can directly measure sine, cosine and tangent.

What happens when the angle, θ, is 0°?
cos 0° = 1, sin 0° = 0 and tan 0° = 0

What happens when θ is 90°?
cos 90° = 0, sin 90° = 1 and tan 90° is undefined

The "sides" can be positive or negative according to the rules of Cartesian coordinates. This makes the sine, cosine and tangent change between positive and negative values also.

Try it yourself:https://www.mathsisfun.com/geometry/unit-circle.html

Animation credit: Charlie Deck