I am trying to find any documentation on what the purpose is of a Zero-Crossing Circuit TRIAC Optoisolator. The datasheets don't explain the concept well enough. If you do answer, please include references or explain how you found out. Thank you!
Zero-crossing is typically used for incandescent bulbs. You may have noticed that when incandescent bulbs fail they always fail when they're switched on. That's because the mains phase can be near its maximum when switching on. Combined with the low resistance of a cold bulb this results in a high current peak, which may burn the filament. When you switch on a zero crossing you avoid these peaks.
How I found out? I've known this since my time in college. It simply makes sense.
Zero Crossing refers to the load voltage at which the triac will switch.
Wikipedia has a little information on the theory of Zero Crossing.
Basically, the triac will hold off its switching until the alternating waveform of the load signal crosses the 'zero' or midpoint of the waveform. This helps alleviate sudden voltage spikes when the switched load jumps from 0V to say 100V in one instant. By ensuring that the load is only switched when it's waveform is crossing the mid-point, the voltage increase will be a smooth rise from 0 to maximum.