For a structure containing the grouping
R_{3}C–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.

Source:

PAC, 1996, *68*, 2193*
(Basic terminology of stereochemistry (IUPAC Recommendations 1996))
* on page 2201