We run into a lot of confusion over these two terms. Many people use them interchangeably, but there is a subtle difference. Both refer to ways of switching the coils of a double-coil humbucking pickup.
Coil Split implies that one coil of the humbucker has been switched or turned off, essentially turning the humbucker into a single-coil pickup. Typically, a split humbucker is not quite as fat sounding as a true single coil, due to the smaller coil size in the humbucker and different magnet design. The output of a split humbucker may not be as hot as a true single-coil.
Coil Tap implies that a portion of the second coil in the humbucker has been removed from the circuit; the coil of wire in the pickup is “tapped” with a switch point somewhere in the middle of the coil. This creates a similar single-coil-type sound, but the result will often be closer to a true single-coil tone than a simple coil split.