"The polarity produced in the molecule by the interaction of two $\mathrm{\pi }$-bonds or between a $\mathrm{\pi }$-bond and lone pairs of electrons present on an adjacent atom." This effect is called

The resonance effect is defined as, the polarity produced in the molecule by the interaction of two $\mathrm{\pi }$-bonds or between a $\mathrm{\pi }$-bond and lone pair of electrons present on an adjacent atom. The effect is transmitted through the chain. There are two types of resonance or mesomeric effects designated as +R and -R or +M and -M respectively.