Mechanical Vignetting
Corner darkening or hard cut-off caused by an accessory physically intruding into the lens's field of view.
In depth
Unlike optical vignetting, it does not improve on stopping down — in fact it sharpens into a distinct dark arc, because the obstruction comes into focus. The usual culprits are stacked filters, a thick filter ring on an ultra-wide lens, or a hood designed for a longer focal length.
Examples in practice
Stacking a polariser on top of a UV filter at 16 mm produces black quarter-circles in every corner.