coterminal anglesThe description of two angles drawn in standard position that share their terminal side.
initial side

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

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°.

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.

terminal sideThe ray that has been rotated around the origin to form an angle with the stationary ray that is the initial side of the angle.
unit circleA circle centered at the origin that has radius 1.