Let’s quickly go over the definition of sine and cosine, and introduce the formal terminology that math books use.

The sine is the result of dividing the side opposite the angle (the height) by the hypotenuse.

The cosine is the result of dividing the
side adjacent to the angle (the base)
by the hypotenuse. *Adjacent is a fancy term
meaning “next to.”*

Here’s the picture again for you to drag.

And, once again, the definitions:

sine = opposite side / hypotenuse

cosine = adjacent side / hypotenuse