In solids especially metals and semiconductors the screening effect reduces the electrostatic field  of an ion inside the solid
in simple words a valence electron is attracted by the nucleus and then repelled by the electrons of the inner shells 
this is known as the screening effect as the electrons make a shield against the valence electron. 

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Screening effect is the ability of the atom to prevent the valence electrons from being pulled towards the nucleus. The screening effect depends only on the number of electron orbits inside the valence shell.
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