Illustrated Glossary of Organic Chemistry

Barrier to rotation: The difference in energy between the most stable and least stable conformations when considering rotation around a bond. It is the energy barrier which must be overcome for one full rotation around a bond.

The barrier to rotation for the C2-C3 bond of 1,3-butadiene is the difference in energies of the s-trans conformation (alkene dihedral angle = 180o; most stable conformation) and the conformation in which the alkenes are perpendicular (alkene dihedral angle = 90o; least stable conformation).