The Rhythm of the Universe.
Plotting trigonometric functions on a coordinate plane to create periodic waves.
Used by Fourier to prove that all sounds can be broken down into sums of simple sine waves.
Sine starts at 0, Cosine starts at 1 (the peak).
In Plain English: If you unzip the Unit Circle and lay it flat, you get a wave. This wave repeats forever. It represents anything that oscillates or cycles.
In The Real World: Audio Engineering. Sound waves, noise cancellation headphones, and music synthesizers all manipulate Sine waves.
Confusing Amplitude (height of the wave) with Period (length of one cycle).