Originally Posted by

**romsek** The ratios of triangle sides and points on the unit circle are equivalent. You can always fit a right triangle between, the origin, the point, and the points projection onto the x-axis.

A more interesting possible definition of sine and cosine is their MacLaurin series.

I wouldn't say there is a "correct" definition, but several equivalent definitions.

However if pressed I'd go with the ratio of right triangle sides as this is generally the first one learned and was probably the first historical definition.