Deceleration is best defined as acceleration that is:

1. Deceleration and negative acceleration are commonly confused. 2. Deceleration always refers to acceleration in the direction opposite to the direction of the velocity. 3. Negative acceleration, however, is acceleration in the negative direction in the chosen coordinate system. 4. A car moving left and speeding up has negative acceleration but is not decelerating. 5. So the two ideas coincide only when the motion happens to be in the positive direction. _Source: OpenStax College Physics for AP(R) Courses 2e (CC BY 4.0), Ch 2 'Kinematics', sections 2.1-2.8_