envelope conformation 

https://doi.org/10.1351/goldbook.E02152
The conformation (of symmetry group Cs) of a five-membered ring in which four atoms are coplanar, and one atom (the flap) projects out of the plane.
Source:
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2208 [Terms] [Paper]
Citation: 'envelope conformation' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.E02152 RIS BibTex EndNote
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Last revised: February 24, 2014