The inverse sine function, also known as arcsin, is a function that takes a number as input and returns the angle, in radians, whose sine is equal to that number. This function is often abbreviated as arcsin because it is the inverse of the sine function, which means that it "undoes" the sine function.
The term "arc" refers to the measure of an angle in radians. An angle is typically measured in degrees, but it can also be measured in radians. One radian is equal to the measure of the central angle of a circle that intercepts an arc on the circle that is equal in length to the radius of the circle.
The term "sin" refers to the sine function, which is a trigonometric function that takes an angle as input and returns the ratio of the length of the side opposite the angle to the length of the hypotenuse of a right triangle.
Therefore, the term "arcsin" can be interpreted as "the arc whose sine is", which refers to the function that returns the angle whose sine is equal to a given number. This function is the inverse of the sine function, so it undoes the operation of the sine function and returns the angle that was originally input into the sine function.