Define Negative Acceleration.

Hint: Acceleration defined as the change in velocity with respect to time is called negative acceleration. We have to think of a type of acceleration which gets lower with respect of time. We have to use an equation to answer this question:
$ \Rightarrow v = u + at$, where $u = $initial velocity,$v = $final velocity.
$ \Rightarrow a = \dfrac{{v - u}}{t} - - (i)$

Complete answer:
We know that acceleration depends on initial velocity $u$and final velocity $v$.
So, from equation$(ii)$ if initial velocity $u$ is less than final velocity$v$,i.e.,$u > v$then acceleration becomes negative.
Hence,
If the velocity of a body decreases with respect to time as the initial velocity gets lower than final velocity due to any force is called Negative Acceleration. Negative acceleration is also called retardation or deceleration.
Examples:
When we apply brakes to a moving vehicle, its velocity gradually decreases and acceleration becomes negative as $u > v$ at this instant.
When we throw a ball in the sky a negative acceleration due to gravity$( - g)$ works in this situation.
When a skateboarder reduces its speed then the acceleration is negative.

Note:
There are $3$equations of motion involve acceleration:
$v = u + at$
$s = ut + \dfrac{1}{2}a{t^2}$
\[{v^2} = {u^2} + 2as\]
 Where $u = $initial velocity, $v = $final velocity, $a = $acceleration, $t = $time taken and $s = $distance travelled.
These equations can be applied only when acceleration is constant i.e., inertial frame of reference otherwise we have to use calculus to solve the question involving non inertial frame of reference.