https://doi.org/10.1351/goldbook.B00673
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–Y eclipses 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. 