Motion in a straight line

Acceleration is defined as the change in velocity per unit time. And this change can occur only if there exists a Force . Also If the change is positive i.e. final velocity is greater than initial velocity then this means that the acceleration is positive i.e. the force which is causing the acceleration is in the same direction of velocity.

But when a ball is thrown upward the gravitational force acting on the ball pulls it down to the earth and that's why it's velocity keeps on decreasing and at a certain height it becomes zero but still at that height gravitational force is pulling the body . So acceleration is still there. And after that ball starts to speed up since force is acting in the same direction of its velocity.

So it's better to define acceleration with respect to force. If force acts then we notice a change in velocity i.e. acceleration and if the force is removed then there will be no acceleration.

Also according to Newton's second law of motion

Acceleration is proportional to the net force acting on the body.

Acceleration is the rate of change of velocity with respect to time. When we press the brakes of a car, that rate is negative. It is the most simple example of velocity in a direction (forward in the case) and acceleration in the other direction.

the right formulation would be: change of velocity in short time is proportional to acceleration. You can drive with constant high velocity and no acceleration. $$ a*Delta t= v_e-v_b$$ at the top your ball has the velocity v_b=0 the acceleration is 10m/s^2 that says, that after 0,1s the velocity will be 1m/s