A smaller version of a humbucking pickup. The coils are the same length as a regular humbucker, but are not as wide. Size-wise, a mini humbucker can be placed between a single-coil and a full-size humbucker. (Actually, a mini humbucker is about the same size as a P-90 pickup.) Tonally, a mini humbucker is brighter than, and not as fat as, a full-size humbucker, but still fatter than a single-coil, and, since it is a humbucker, there are no problems with noise or hum.
A variety of guitars from Gibson (such as the Firebird and the Les Paul Deluxe), Epiphone (several models), and other manufacturers have featured mini humbuckers.