Students interpret a positive acceleration as always meaning "speeding up" and a negative acceleration as "slowing down", rather than associating the sign with the direction of the acceleration vector. This is a difficult idea to change, and for many students it becomes a serious difficultly when they get to Newton's second law.
The sign of the acceleration indicates the direction of the acceleration vector. However, it does not indicate how the speed of the object will change. When the acceleration vector is parallel to the velocity vector, the speed of the object will increase. Conversely, when the acceleration vector is anti-parallel (in opposite directions) to the velocity vector, the speed of the object will decrease.
According to Physics for Scientists and Engineers (6th edition) by R. Serway and J. Jewett,
The word deceleration has the common popular connotation of slowing down. We will not use this word in this text, because it further confuses the definition we have given for negative acceleration.
In other words, deceleration is NOT equivalent to negative acceleration.
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