In trigonometry, the sine of an angle equals the y-coordinate of the point where a line drawn at that angle through the center of a coordinate grid intersects a unit circle.
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In general, it is defined (in a right triangle) as the ratio of the length of the side opposite the angle to the length of the hypotenuse; in the unit circle diagram, however, the hypotenuse is always one unit in length.
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The name is derived from the Latin word sinus, meaning "curve". The sine of a number is just another number, of course, but when the sine function is graphed on a coordinate graph, it produces a smooth, rolling ("sinusoidal") curve, hence the name.