Trigonometric functions are a type of function in math that deal with angles. They connect the measurement of an angle with the ratios of the sides of a right triangle containing that angle, and they are a basic tool for studying triangles and periodic phenomena.
A ring, also called an annulus in geometry, refers to the part left when a smaller concentric circle is cut out of a larger circle, basically like a hollow circle.
Approximate equality is a symbol used in math to represent numbers that are close in value. It's written as '≈' and read as 'approximately equal to'.