a straight line that intersects a curve at two or more points
The trigonometric functions (also called the circular functions) are functions of an angle. They are used to relate the angles of a triangle to the lengths of the sides of a triangle. Trigonometric functions are important in the study of triangles and modeling periodic phenomena, among many other applications.
The most familiar trigonometric functions are the sine, cosine, and tangent. In the context of the standard unit circle with radius 1, where a triangle is formed by a ray originating at the origin and making some angle with the $x$-axis, the sine of the angle gives the length of the $y$-component (rise) of the triangle, the cosine gives the length of the $x$-component (run), and the tangent function gives the slope ($y$-component divided by the $x$-component).
With this in mind, we can use the definition of a derivative to calculate the derivatives of different trigonometric functions: