In many different areas of geometry, you will utilize something called an Inverse Trigonometric Function, which is simply the inverse function of the corresponding trigonometric function.
Arcsine, or $\arcsin$ is the inverse function of the sine function. It is defined on the domain $[-1, 1]$, and its range (in radians) is $[-\frac{\pi}{2}, \frac{\pi}{2}]$. It is defined as:
$$ \arcsin{\theta} = \sin^{-1}{\theta} $$Arccosine, or $\arccos$ is the inverse function of the cosine function. It is defined on the domain $[-1, 1]$, and its range (in radians) is $[0, \pi]$. It is defined as:
$$ \arccos{\theta} = \cos^{-1}{\theta} $$Arctangent, or $\arctan$ is the inverse function of the tangent function. It is defined across all real numbers, and its range (in radians) is $[-\frac{\pi}{2}, \frac{\pi}{2}]$. It is defined as:
$$ \arctan{\theta} = \tan^{-1}{\theta} $$Arccosecant is the inverse function of the cosecant function. It is defined across $x \leq -1$ and $x \geq 1$, with the range of $[-\frac{\pi}{2}, 0)$ and $(0, \frac{\pi}{2}]$.
Arcsecant is the inverse function of the secant function. It is defined across $x \leq -1$ and $x \geq 1$, with the range of $[0, \frac{\pi}{2})$ and $(\frac{\pi}{2}, \pi]$.
Arccotangant is the inverse function of the cotangent function. It is defined across all real numbers, with the range of $(0, \pi)$.