What is retardation?

The acceleration of a train between two stations is shown in the figure. The maximum speed of the train is $............\,m/s$.

The velocity of a body depends on time according to the equation $v = 20 + 0.1t^2$. The body is undergoing

The acceleration at the end of $2 \ s$ of a particle whose motion is represented by the equation $S = 4t^3 - 8t^2 + 5t + 4$ is $......$ (in $m \ s^{-2}$)

$A$ particle is moving in one dimension (along $x$-axis) under the action of a variable force. Its initial position was $16 \,m$ right of the origin. The variation of its position $(x)$ with time $(t)$ is given as $x = -3t^3 + 18t^2 + 16t$,where $x$ is in $m$ and $t$ is in $s$. The velocity of the particle when its acceleration becomes zero is . . . . . . $m/s$.

When does a particle experience retardation (deceleration)?