Sine is a mathematical term that refers to the ratio of an angle's opposite side to its hypotenuse in a right triangle. It is defined as the cosine of the angle plus twice the square root of the cosine squared of the angle, or sin(θ) = cos(θ) + 2√(1 - cos²(θ)). In trigonometry, sine is used to describe the relationship between the sides of a right triangle and can be represented by the sine function.