Delocalisation
The spreading of electrons over three or more atoms rather than their confinement between two.
Unit
Dimensionless
In depth
Delocalisation requires a continuous array of parallel p orbitals and always lowers the energy, because electrons confined to a larger region have lower kinetic energy. It equalises bond lengths, stabilises intermediates such as carboxylate and allyl ions, and shifts absorption to longer wavelengths as conjugation extends.
Examples in the real world
Benzene's six pi electrons spread over the whole ring; the equal C–O bonds of the acetate ion.