What term describes stereoisomers that differ in the rotation about a single sigma bond?

The term that describes stereoisomers differing in rotation about a single sigma bond is conformational isomers or conformers. These variations arise from the rotation around single bonds, which allows the molecule to adopt different spatial arrangements without breaking any bonds.

Conformers can interconvert relatively easily and do not require the addition of energy to overcome barriers, unlike configurational isomers, which require bond-breaking to change from one form to another. For instance, in the case of butane, the different arrangements of atoms around the C-C single bond lead to distinct conformers like eclipsed and staggered forms.

Geometric and optical isomers cater to different aspects of stereochemistry. Geometric isomers typically refer to differences in spatial arrangement around double bonds or rings due to restricted rotation, while optical isomers are related to the chiral nature of molecules and their ability to rotate plane-polarized light. Thus, these terms do not accurately describe the specific nature of alterations happening through sigma bond rotation.