In bowling, a spilt is a situation in which most of the pins have been knocked down, and those remaining standing are separated by one or more empty spaces. Spits most often occur when the bowler has thrown the ball too fast, or without enough hook.
Splits are by far the most difficult spares to convert. For those where one of the pins is positioned far forward of the other (for example, a 3-10 or a 2-7) the best strategy is to aim between the two. When the pins are closer to parallel, the spare becomes harder to make, and the best bet is to hit the forward pin on its side and hope it slides into the other.
Splits in which the pins are parallel are nearly (though not entirely!) impossible to convert into spares. The worst is the notorious 7-10 split, in which the two rear corner pins (look here for a diagram) remain, separated by two empty pin-spaces. An outside approach is difficult because of the gutter to the outside of each of these pins. I've never even seen one converted successfully in regular tenpin bowling, but I've done so myself in duckpins, where the much lighter pins can be made to fly crosswards more easily.