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