The interaction between π bond and lone pair of electrons present on an adjacent atom is responsible for

Resonance Effect – The polarity induced in a molecule by the interaction of a lone pair of electrons with a pi bond or the interaction of two pi bonds in nearby atoms is known as the resonance effect.

The inductive and resonance effect can be effectively employed to explain the stability of carbocations, free radicals and carbanions and the acidic and basic strengths of various organic compounds.