It's also a matter of definition.
In a correctly executed turn, in balance, the tail flies through the exact same piece of air as the nose.
If (in a left turn) you have too much left rudder (balance ball is right), the nose is too much inwards and the nose follows a trajectory with a shorter radius than the tail. The aircraft moves sideways, to the right, through the air. This is called skidding out of the turn.
If (in a left turn) you have not enough left rudder (balance ball is left), the nose is too much outwards and the nose follows a trajectory with a larger radius than the tail. The aircraft moves sideways, to the left, through the air. This is called slipping into the turn.