Because that is accepted definition. The sine is opposite over hypotenuse, or Y in the unit circle. The cosine is adjacent over hypotenuse, or X in the unit circle. The tangent is sine over cosine, etc. For more information, please see the related link below.
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It is the sine ratio.
It is the Sine(Sin) ratio. Sin(angle) = o/h
If you have a right triangle, the sine of an angle is the ratio of the opposite side to the hypotenuse.
This ratio is called the sine ratio for a right triangle. A simple way to remember this is by using the mnemonic 'SOHCAHTOA', where SOH stands for sine equals the ratio opposite side/ hypotenuse, COH is cosine equals adjacent side/hypotenuse, and TOA is tangent equals opposite side/adjacent side.
Sine ratio = opposite/hypotenuse