Any time the ball is not in the center you are in a slip.
Nope, the ball can be in the centre with differential thrust, and the aircraft is side slipping. When the rudder is applied to counter the yaw from the differential thrust, it applies a side force to the tail and that force not only supplies the opposite torque to counter the thrust differential, but also applies a force which accelerates the aircraft sideways until the side force of the rudder is matched by fuselage drag. Even though the wings are level, and the ball is in the middle, the aircraft is tracking somewhat to the side of the longitudinal axis.
If you are just banking into the crosswind, you can find a point where your gentle banking turn doesn't actually turn, and the resultant vector of flight is along the runway heading.
No, any turn is
by definition a turn - the heading is changing!
I think with this interpretation you see an uncoordinated turn,
... and the aircraft is slipping (or skidding) in an "uncoordinated turn".
