Radians: A unit of measurement for angles derived from the radius of a circle. One radian is equal to the angle subtended at the center of a circle by an arc that has the same length as its radius.
Trigonometric Functions: Mathematical functions (such as sine, cosine, and tangent) that relate angles of a right triangle to ratios between its sides.
Unit Circle: A circle with a radius of 1 unit, centered at the origin (0, 0), often used to understand and define trigonometric functions.