From Wikipedia
The P-90 (sometimes written P90) is a single-coil electric guitar pickup produced by Gibson Guitar Corporation since 1946, as well as other vendors. Compared to other single coil designs, such as the Fender single coil, the bobbin for a P-90 is deeper but narrower. The Fender-style single-coil is wound in a taller bobbin, but the wires are closer to the individual poles. This makes the P-90 produce a tone seen as a midpoint for pickups: "Sharper" than humbuckers, but with less top-end than single-coils. As with other single-coil pickups, the P-90 is subject to AC-hum unless some form of cancelation is used.