Waviness is a quality of being curly, or of curving back and forth, like the waviness of your sister's hair or the waviness of the lines in an abstract painting.
Waviness is all about waves, which can be moving in an undulating way or just have that wavy shape. So you can use the word for hair with a little curl — whether it's the waviness of your new hair style or your poodle's swirly fur — but also for the moving surface of ocean water. You might sit in a sailboat and admire the waviness of the sail that's blowing in the wind and the waviness of the water below.