Definition of split
A split is when a pot is separated. There are two reasons for splitting a pot: when 2 players have the same hand at the showdown or when playing a "high-low" variant, that is, a variant in which you often play for two halves of the pot, half of which you win when you make the best high and half of which you win when you make the best low. In high only variants, if 2 or more players have the same hand, the pot is split equally. A split pot can also occur when the board gives everyone the best hand (for example, if a board gives T-J-Q-K-A and there are no flushes).
What if the prize pool is not divisible by 2?
Let's say you are playing heads-up with a player. You come to the showdown on a 2-2-5-5-7 board and both of you have AK. This is a split pot. But the pot is $10.11, a number that doesn't split in half (actually it does, but $0.01 is the smallest unit of currency). What do we do? We deal 5.05 to each player and the excess ($0.01) goes to the player who was out of position postflop.