Cosine

Cosine is a trigonometric function describing the ratio of the adjacent side to the hypotenuse in a right triangle. It is often abbreviated cos.

x values y values 0 1 π/2 0 π -1 3π/2 0 2π 1

That would be a full period of the graph - as 2π radians is equivalent to one circle. The graph continues in that pattern.

Relation to secant

$\cos \theta =\sin \left({\frac {\pi }{2}}-\theta \right)={\frac {1}{\sec \theta }}\,$