What is the rate of deceleration expressed as?

Rate of deceleration is expressed as negative acceleration. The negative sign indicates that deceleration takes place in the opposite direction as acceleration. Both are as expressed in standard units m/s^2.

Acceleration is the rate of change of velocity. When the velocity changes due to an increase in velocity, the acceleration is occurring in the same direction as the motion of the object, so it is given a positive value. If the acceleration occurs in the opposite direction as the motion of the object, that indicates that the velocity of the object is decreasing over time. In such cases, acceleration (or deceleration) is given a negative value.