Pickups wired so that the finish lead (negative or ground) of one coil is joined to the start lead (
Hot) of another coil so that the start of one coil and finish of another coil become the terminal connections for the pickup(s). Typically resulting in higher resistance (more
Output) and a tone that is
Beefy,
Solid, with more low end but with less high frequency response, less clarity, and less softness.