Two atoms or groups attached to adjacent atoms are said to be eclipsed if the torsion angle between the three bonds is zero (or approximately so).
See: bisecting conformation, eclipsing strain
Source:
PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2206 (https://doi.org/10.1351/pac199668122193)