Linked from
The 27 pages that link to Sine, each with the reason it gives.
Right triangleBroader topic: It measures one leg relative to the hypotenuse for either acute angle.
TrigonometryBroader topic: It is the opposite-to-hypotenuse ratio in a right triangle.
Unit circleBroader topic: The point's height at angle θ is sin θ.
AryabhataBroader topic: Aryabhata gave a table of half-chords that corresponds to sine values.
Range of a functionBroader topic: Its oscillation attains every value from negative one to one.