Acceleration is the variation of the velocity with time.

The velocity is a vector, so it has a magnitude and a direction. If you keep the same velocity and change the direction you accelerate. Slowing down the magnitude of the speed before the turn compensates for the change in the direction that you are going to have in the turn. This change in direction is an acceleration. Half way around the turn you can start to go back to the original magnitude of the speed in new direction by accelerating.