What is resonance?

Answer 1

It is the frequency in an electric circuit at which the inducive and capacitive reactances sum up to 0.

Doug

Answer 2

Resonance has to do with the bond structure where an electron is not specifically in a bond between two atoms. An example is a compound such as acetic acid.
................................. CH3-(CO)OH. The carboxyl carbon has a double bond with O and a single bond with the hydroxyl O. This is accomplished by three "hybrid" sp2 bonds and a fourth atomic p-bond orbital. The oxygen has similar bonds. One of the sp2 bonds is with C-C, the second is the one of two bonds with O, and the third is a bond with the -OH group. The double bond is formed with the p-orbitals of C and O. The acetate ion is formed when the hydrogen from the OH group ionizes, leaving a negative charge on the acetate ion. This charge is stabilized by resonance between the carbon and the two oxygens, which are now "equal" in terms of their bonding with carbon. Resonance is important in all the oxy-acids, such as nitric acid, which is
HO-(NO)=O, where the nitrogen has a double bond with one oxygen and a coordinated bond with the other, and HO-(SO2)-OH, sulfuric acid.

Resonance is extremely important in the "aromatic" hydrocarbons of which benzene (c6h6) is the base. The compound is has a flat hexagonal shape, where each carbon has a sp2 hybrid orbital and a p orbital. One of the sp2 orbitals bonds each C to an H. The second and third orbitals bond each C singly to its adjacent C's. This leaves 3 electrons to enter into bonds. They resonate in the p orbitals, whose shapes allow the 3 electrons to de-localize into a "cloud" on either side of the C6H6 molecule.

Answer 3

Both replies are right, you should specify a bit your question since resonance means at least 2 different things

Ilusion