The electron displacement in an organic molecule may occur either in the ground state under the influence of an atom or a substituent group or in the existence of an appropriate attacking reagent.
The electron displacements due to the influence of an atom or a substituent group existing in the molecule cause permanent polarization of the bond.
Inductive effects and resonance effects are some examples of electron displacement.
Temporary electron displacement effects are visible in a molecule when a reagent approaches to attack it.
This type of electron displacement is commonly known as the electromeric effect or polarisability effect.