What is meant by the term retardation? Name its S.I. unit.

If the velocity of a body decreases with time, it is called retardation. As it is decrease in velocity per second, so, retardation is negative acceleration.

It's unit is same as that of acceleration.

$\text{S.I. unit of retardation} = \dfrac{\text{meter}}{\text{second}^2} = \dfrac{\text{m}}{\text{s}^2}\\[0.5em] \text{C.G.S. unit of retardation} = \dfrac{\text{centimeter}}{\text{second}^2} = \dfrac{\text{cm}}{\text{s}^2}\\[0.5em]$