In a multielectron atom, the electrons of the valence shell are attracted to the nucleus and also these electrons are repelled by the electrons present in the inner shells. On account of this, the actual force of attraction between the nucleus and the valence electrons is somewhat decreased by the repulsive forces acting in opposite directions.
This decrease in the force of attraction exerted by the nucleus on the valence electrons due to the presence of electrons in the inner shells is called screening effect or shielding effect.
The magnitude of the screening effect depends upon the number of inner electrons higher the number of inner electrons greater shall be the value of the screening effect. The screening effect constant is represented by the symbol σ.