Co-Witness
Aligning a red dot with backup iron sights so both can be used together — either with the irons centered in the dot (absolute) or lower (lower 1/3).
Co-witnessing means mounting a red dot at a height where your backup iron sights line up through the optic's window. 'Absolute co-witness' puts the irons dead-center on the dot; 'lower 1/3 co-witness' raises the dot so the irons sit in the bottom third of the window, giving a cleaner sight picture while keeping irons available.
The point is redundancy: if the electronic dot fails, you can still aim through the same window using the irons without changing anything. It also provides a quick way to confirm your dot hasn't drifted. Which style you choose comes down to preference and how much of the window you want unobstructed.