The rate of change of velocity with time of a moving body can either be positive or negative. If rate of change of velocity with time is positive, it is called acceleration and if negative, it is called retardation.
That means, negative acceleration is called retardation.
Retardation is measured as an amount by which the velocity of a body changes in one second. The unit and dimension art retardation is the same as that of acceleration. If the velocity of a body is decreasing, then its acceleration will be negative. The negative acceleration is also called retardation or deceleration.
Example: When something thrown upward the acceleration due to gravity acts downward and taken as negative. When apply break to stop cycle then also acceleration is negative.