What is thermal isomerization?
The term "thermally" is used here to contrast with a photochemical isomerization. In the chemistry of vision, 11-cis retinal absorbs light and is isomerized to the all-trans isomer. This is a photochemical process because it involves the absorption of a photon to promote one electron into an antibonding orbital. In this excited state, the geometry can rearrange to the more stable all-trans form before the electron relaxes back to the ground state.

In a "thermal" process, no photons are absorbed, no electrons are promoted, and no excited states are needed. The molecule takes advantage of ambient or supplied kinetic energy to reorganize.