Beat envelope detection
If you add two cosines you simply get the product of the sum and difference frequencies.
$$cos(x) + cos(y) = 2\cdot cos \left( \frac{x+y}{2} \right) \cdot cos \left( \frac{x-y}{2} \right)$$
If the frequencies are very close together, than the difference frequency is close to zero and that's what creates the &quot;beat&quot;.
The exact definition of what constitutes a beat and what doesn't depends on your application. For example: in audio I would call any frequency difference of less than maybe 4 Hz or so a beat since it creates the typical beat sound.