Resonance effect or mesomeric effect
(1) The effect where electrons are transferred from a multiple bond to a molecule, or from a multiple bond to a one covalent bond or lone pair or pairs of electrons from an atom to the adjacent single covalent bond is known as mesomeric effect or just as M-effect. In type of the element along with conjugated system of double bonds, the mesomeric effect is transferred via whole of the conjugated system and thus the effect may better be termed as conjugative effect.
(2) Groups that have the capacity to raise the electron density of the rest of the molecule are said to have +M effect. Such types of sets act as lone pairs of electrons. Sets that decrease the electron density of the rest of the molecule by withdrawing electron pairs are supposed to have -M effect.
(3) both of the inductive and mesomeric effects, while given together, can behave in similar direction or oppose one another. The mesomeric effect is harder as compared to the former. As given, in vinyl chloride because of - I effect the chlorine atom should expand a negative charge but on account of mesomeric effect it comprise positive charge.