Iron sight alignment in optic glass
Co-witnessing sights is commonly defined as being able to align the iron sights (front and rear sights) through the optic. Location does not matter; the sights are usually visible in the lower third of the optic glass. When the irons are aligned in the lower third of the optic view, it is referred to as a 1/3 co-witness.
Another common use of the term (referred to as absolute co-witness) involves zeroing the optic, and then bringing the iron sights to align with the optic's reticule, producing the same zero on the irons.