What is meant by "elastic collisions"?

Elastic collisions meant that there will be no change in the total momentum of the system and surrounding after collision.

Explanation:

Collision is the process where two or more bodies hit each other. So when they hit each other, transfer of energy will be occurring between them. In other words, the momentum can be changed after the collision. There are two kinds of collision.

The elastic collision are those which will not undergo any change in the total momentum after collision. This means that the sum of momentum of the objects before collision will be equal to the sum of momentum of the objects after collision. Thus, there will be no change in the total momentum of the object after collision in case of elastic collision.

radioactive decay occurs in unstable atomic nuclei – that is, ones that don't have enough binding energy to hold

the nucleus together due to an excess of either protons or neutrons.