Sometimes seen in the specifications of electric guitars, and often used interchangeably (incorrectly!), coil-splitting and coil-tapping provide a means of increasing the range of sounds you can get from an electric guitar pickup. These functions are often activated by pulling out the volume or tone controls.
Coil-splitting applies to humbucking pickups and allows you to use only one of the two coils, giving you a single-coil sound from a humbucking pickup. Coil-tapping is used to reduce the output of a single-coil pickup by ‘tapping’ the coil of wire meaning that the sound is coming from fewer windings of the pickup coil; this can often produce a more ‘vintage’ sound.