Contributing resonance structures: One or more alternate Lewis structures for a molecule or ion.
Double-headed arrow: Drawn between two molecular structures to indicate that they are contributing resonance structures.
Resonance: When a molecule can be represented by the weighted hybrid of two or more hypothetical but reasonable Lewis structures that differ only in the distribution of bonding and nonbonding electrons, and in which the positions of the nuclei are constant.
Resonance hybrid structure: A structure which is the weighted average of all contributing resonance structures for a molecule or ion. The resonance hybrid structure is the closest representation of reality for that molecule or ion.