If A is an acute angle of a right triangle, then the cosine of angle A is the ratio of the length of the side adjacent to angle A over the length of the hypotenuse.

The stationary ray that forms an angle in standard position and lies on the positive x-axis.

The formula that relates the lengths of the sides of any right triangle: , where c is the hypotenuse, and a and b are the legs of the right triangle.

The angle formed by the terminal side of an angle in standard position and the x-axis, whose measure is between 0° and 90°.

A mirror-image of a graph. If the reflection is over the x-axis, then the part of the original graph that was below the x-axis will be above the x-axis, and vice versa.

If A is an acute angle of a right triangle, then the sine of angle A is the ratio of the length of the side opposite angle A over the length of the hypotenuse.

The placement of an angle upon a set of coordinate axes with its vertex at the origin, its initial side placed along the positive x-axis, and a directional arrow pointing to the angle’s terminal side.

The left and right halves of the graph are mirror images of each other over the y-axis.

If A is an acute angle of a right triangle, then the tangent of angle A is the ratio of the length of the side opposite angle A over the length of the side adjacent to A.

adjacent side	For a given acute angle in a right triangle, the adjacent side to that angle is the side that, along with the hypotenuse, forms that acute angle.
amplitude	Half the difference between the maximum and the minimum values of a periodic function.
central angle	An angle whose vertex is at the center of a circle.
cofunctions	Two trigonometric functions, such as sine and cosine, for which the value of the first function at an acute angle equals the value of the second function at the complement of that angle.
cosine	If A is an acute angle of a right triangle, then the cosine of angle A is the ratio of the length of the side adjacent to angle A over the length of the hypotenuse.
coterminal angles	The description of two angles drawn in standard position that share their terminal side.
cycle	Any part of a graph of a periodic function that is one period long.
hypotenuse	The side opposite the right angle in any right triangle. The hypotenuse is the longest side of any right triangle.
identity	An equation that is true for any possible value of the variable.
initial side	The stationary ray that forms an angle in standard position and lies on the positive x-axis.
inverse function	If you take a function and reverse its inputs and outputs, then you get its inverse function.
opposite side	For a given acute angle in a right triangle, the opposite side to that angle is the side that is not one of the two sides that form that acute angle.
period	The length of the smallest interval that contains exactly one copy of the repeating pattern of a periodic function.
periodic function	A function whose graph has a pattern that repeats forever in both directions.
Pythagorean Theorem	The formula that relates the lengths of the sides of any right triangle: , where c is the hypotenuse, and a and b are the legs of the right triangle.
radian measure	A measure of a central angle given by the ratio of the arc length to the radius.
reference angle	The angle formed by the terminal side of an angle in standard position and the x-axis, whose measure is between 0° and 90°.
reflection	A mirror-image of a graph. If the reflection is over the x-axis, then the part of the original graph that was below the x-axis will be above the x-axis, and vice versa.
sine	If A is an acute angle of a right triangle, then the sine of angle A is the ratio of the length of the side opposite angle A over the length of the hypotenuse.
standard position	The placement of an angle upon a set of coordinate axes with its vertex at the origin, its initial side placed along the positive x-axis, and a directional arrow pointing to the angle’s terminal side.
symmetric about the y-axis	The left and right halves of the graph are mirror images of each other over the y-axis.
tangent	If A is an acute angle of a right triangle, then the tangent of angle A is the ratio of the length of the side opposite angle A over the length of the side adjacent to A.
terminal side	The ray that has been rotated around the origin to form an angle with the stationary ray that is the initial side of the angle.
trigonometric functions	A function of an angle expressed as the ratio of two of the sides of a right triangle that contains that angle; the sine, cosine, tangent, cotangent, secant, cosecant.
unit circle	A circle centered at the origin that has radius 1.