envelope conformation

https://doi.org/10.1351/goldbook.E02152
The @C01258@ (of symmetry group \(C_{\text{s}}\)) of a five-membered ring in which four atoms are coplanar, and one atom (the flap) projects out of the plane.
E02152.png
Source:
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2208 [Terms] [Paper]