A Yamaha Active Single Coil Pick Up. They are part of the Yamaha's H.I.P.S (Hybrid Integrated Pickup System) this system makes it possible for all pickups to be switched on or off individually. The Humbucker version can also be switched to single-coil operation. On some guitar models where this pickup is used it is even possible to switch them from active to passive and vice versa.
They are sometimes compared to EMG pickups but are said to be less sterile.