In a stereochemical context the rotation about a bond is called '
free' when the
rotational barrier is so low that different
conformations are not perceptible as different chemical species on the time scale of the experiment.
The
inhibition of rotation of groups about a bond due to the presence of a sufficiently large
rotational barrier to make the phenomenon observable on the time scale of the experiment is termed hindered
rotation or restricted rotation.
Source:
PAC, 1996, 68, 2193
(Basic terminology of stereochemistry (IUPAC Recommendations 1996))
on page 2209