bisecting conformation (eclipsing conformation)

For a structure containing the grouping R3C–C(Y)=X (with identical or different groups R ) the conformation in which the torsion angle is such that X is antiperiplanar to one of the groups R , and, in a Newman projection, the double bond C=X bisects one of the R–C–R angles. In this conformation the bond C–Yeclipses one of the C–R bonds. The other conformation, in which X is synperiplanar to one of the groups R , is called an eclipsing conformation.
PAC, 1996, 68, 2193 (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2201