In chemistry, the term resonance is used to describe instances where the actual electronic distribution in a molecule is a combination of several contributing structures. The contributing structures are known as resonance structures or resonance forms.
Examples of common resonance structures
A carboxylate anion is written with a double bond between carbon and one oxygen atom and a single bond between carbon and the other oxygen atom, which bears a negative charge. But the other resonance structure is equally correct. The real structure of a carboxylate ion is an average of the two.
A benzene ring (or any aromatic compound) can be written as two different resonance structures, placing the double bonds in the ring in two different ways. In reality, the structure of benzene is an average of the two structures, with each bond being in between a single and a double bond.
A protonated carbonyl compound is written with a double bond between carbon and oxygen and a formal charge on oxygen, but the other resonance structure, placing the formal charge on carbon and having only a single bond between carbon and oxygen is equally correct. In reality, the real structure is somewhere in between.