Torsional strain is the resistance to bond twisting. In cyclic molecules, it is also called Pitzer strain.
Torsional strain occurs when atoms separated by three bonds are placed in an eclipsed conformation instead of the more stable staggered conformation.  The barrier of rotation between staggered conformations of ethane is approximately 2.9 kcal mol. It was initially believed that the barrier to rotation was due to steric interactions between vicinal hydrogens, but the Van der Waals radius of hydrogen is too small for this to be the case. Recent research has shown that the staggered conformation may be more stable due to a hyperconjugative effect.  Rotation away from the staggered conformation interrupts this stabilizing force.
More complex molecules, such as butane, have more than one possible staggered conformation.  The anti conformation of butane is approximately 0.9 kcal mol (3.8 kJ mol) more stable than the gauche conformation.  Both of these staggered conformations are much more stable than the eclipsed conformations.  Instead of a hyperconjugative effect, such as that in ethane, the strain energy in butane is due to both steric interactions between methyl groups and angle strain caused by these interactions.