Two or more structures of a molecule or ion with identical arrangements of the atoms but different arrangements of the electrons. One or more electron pairs are involved with more than one atom in the process of forming bonds.
In the example of an aromatic hydrocarbon, the real molecule is a resonance hybrid made up of multiple contributing resonance structures. It behaves as an average of the contributing structures, with the negative charges spread evenly over the carbon ring. The molecule does not physically alternate between the multiple possible structures - it is a stable hybrid of the contributing structures.