Negative acceleration is called retardation. If the velocity of a body is decreasing with respect to time, the acceleration is said to be negative.

Negative acceleration or deceleration
The negative acceleration is known as retardation.

Retardation means negative acceleration
Declaration
negetive acceleration

When speed of object decreases with time then then it is called retardation
When the initial velocity is more than the final velocity, the acceleration gets a negative sign. This is also called retardation.
Retardation is the acceleration in opposite direction. Car when applies brakes we say it is retarding
Retardation is actually acceleration with the negative sign .
Acceleration in negative