The resonance effect definition can be given as a chemical phenomenon, observed in the characteristic compounds containing double bonds in the organic compounds. The organic compounds have these double bonds in the structures and have the overlapping of the p-orbitals, usually on the two adjacent sides of carbon atoms.

Negative Resonance Effect: The Negative resonance effect happens when the groups withdraw the electrons from the other molecules by the delocalisation process. Usually, the groups are denoted either by -R or -M. The molecular electron density is said to decrease in this process. The negative resonance effect examples are, $\mathrm{C}=\mathrm{O}, -\mathrm{COOH}, -\mathrm{C}\equiv \mathrm{N}$ and $-{\mathrm{NO}}_2$.

$-\mathrm{COOH}$ shows $–\mathrm{R}$ effect, while rest $3$ groups shows $+\mathrm{R}$ effect via lone pair.