A "pericyclic reaction" is an organic reaction that proceeds via a single concerted and cyclic transition state, the geometry of which allows for the continuous overlap of a cycle of (π and/or σ) orbitals. In the language of orbital symmetry, a pericyclic reaction is termed "symmetry-forbidden" if there is an additional symmetry-imposed energetic barrier arising from the intended correlation of the ground state electron configuration of the starting material with an excited state electron configuration of the product and vice versa. (Although the non-crossing rule forbids such a correlation, the rise in energy as the intended crossing is approached results in an additional energy barrier nonetheless.) A pericyclic reaction is classified as "symmetry-allowed" if no such symmetry-imposed barrier exists. Thus, these terms do not imply whether a reaction in question will actually take place. Rather, with all other energetic factors being equal, a symmetry-forbidden process will be impeded by an additional energetic barrier. Although the symmetry-imposed barrier is often formidable (up to ca. 5 eV or 480 kJ/mol in the case of a forbidden [2+2] cycloaddition), the prohibition is not absolute, and symmetry-forbidden reactions can still take place via a pericyclic pathway if other factors (e.g. strain release) favor the reaction. Likewise, a symmetry-allowed reaction may be preempted by an insurmountable energetic barrier resulting from factors unrelated to orbital symmetry.