What is a sine wave?
One way would be to describe a sinewave with respect to the unit circle.  The radius obviously draws a circle BUT the x and y co-ordinates trace out the familiar waveforms. 

This also helps with pictorially explaining Eulers formula:

\$e^{i x} = cos(x)+ i\cdot sin(x)\$

where the special case of \$x = \pi\$ yields Eulers identity: 
\$e^{i \pi} + 1 = 0\$


(source: https://betterexplained.com/articles/intuitive-understanding-of-sine-waves/)