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.

                 |
            _____|_____     _
           /     |   /|\     \
          /      |  / | \     } sine
         |       | /  |  |  _/
_________|_______|/___|__|_________
         |       |       |
         |       |       |
          \      |      /
           \_____|_____/
                 |
                 |

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.

                    /|
                   / |
                  /  |
     hypotenuse  /   |  opposite
                /    |
               /     |
              /     _|
             /_____|_|
        angle

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.