Beats are a phenomenon that occurs whenever multiple sinusoids are added together. The exact definition is hard to explain since there are two recurring types of these phenomena.
In one of these types, they can be described as a wave that tangents every period of a signal comprised of sinewaves. In my oldest online description of this type of beat I coined them to be perfect. I had to make this distinction because of another type.
The other type could be described as imperfect. If you play around with my Dissonance calculator, you will quickly discover them by yourself.
A property of beats that is really neat, is the property that when you add the same variable to each frequency in a signal, you will be left with a wave with the same initial beat function. Here is an animation that shows this property, the number at the top is the variable that is being added.
This is also the property in the calculator labeled “Enhancement amount”.
This is also a good example of what a perfect beat looks like (ctrl+R to sync the videos).