Mesomerism

Mesomerism is a way of representing the actual structure of a molecular species with delocalised bonding electrons; in this method, the species is considered to be intermediate between two or more hypothetical structures having localised bonds, i.e. able to be represented by the usual Lewis formulae.

Note:
1. From the Greek mesos, "in the middle".
2. This method of representation was considered to be the expression of the concept of resonance later developed using quantum mechanics.
3. Contrary to what is sometimes said, the limiting structures cannot be called "mesomers".
See also: conjugation, resonance.