A stacked humbucker is a humbucking pickup that is compacted to fit into a single-coil route by vertically stacking the coils, rather than placing them next to each other horizontally as with a standard humbucker. This allows for drop-in replacement of single coil pickups with humbucking pickups.
The single-coil form factor retains some of the high-end clarity of a single-coil pickup, while adding much of the girth of a humbucker, and, of course, the hum canceling ability. the narrower magnetic aperture of the pickup compared to a side-by-side humbucker also contributes to a tone that is somewhere in between a true single-coil and a “regular” humbucker.