Two atoms or groups attached to adjacent atoms are said to be eclipsed if the @T06406@ between the three bonds is zero (or approximately so).
Source: PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2206 [Terms] [Paper]
Citation: 'eclipsed (eclipsing)' in IUPAC Compendium of Chemical Terminology, 3rd ed. International Union of Pure and Applied Chemistry; 2006. Online version 3.0.1, 2019. https://doi.org/10.1351/goldbook.E01884 RISBibTexEndNote