What is the quantum effect?

Hint: In Physics we have different branches to study different levels of physical phenomena. The study of massive objects and Newton’s laws are part of Classical physics. But when we want to know what is happening inside the atoms, inside the nucleus, classical physics fails to explain this. From here comes into the picture called quantum physics and all the phenomena happening at this small scale is called the quantum effect.

Complete answer:
The laws of quantum physics are valid and acting even on the projectile moving but we make use of classical physics here to make our calculations easy. That is why we call it a non-quantum system. A quantum system is one where classical laws do not work and quantum behavior becomes too obvious.
So now, if we define what quantum effect is, we can say that quantum effect is an effect that is properly predicted or explained by quantum physics but not by classical physics.

Note:
In classical physics, we explain the composition of matter by solid particles but in quantum physics, we explain the same in terms of the wave.
So, whenever we discuss the matter as a wave, we study quantum mechanics or we are discussing the quantum effect. Although the study of sound waves is not counted as a quantum effect because here the particles are still a solid ball. so, to show the quantum effect, the particle must behave as a wave.
On a large scale, quantum mechanics is still valid but the only reason for not using it is its operation is not obvious.