The six trigonometric functions are sine, cosine, tangent, cotangent, secant and cotangent. In mathematical analysis, trigonometric function is also defined as the solution of infinite series or specific differential equation, which allows its value to be extended to any real value or even complex value.

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Trigonometric function is one of the important contents of middle school mathematics. Trigonometric function is a kind of transcendental function in elementary functions of mathematics. Their essence is the mapping between the set of arbitrary angles and a set of ratio variables.

The usual trigonometric function is defined in the plane rectangular coordinate system, and its domain is the whole real number domain. The other is defined in a right triangle, but it is incomplete. Modern mathematics describes them as the limit of infinite sequence and the solution of differential equation, and extends their definitions to complex system.