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.
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Source:
PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2208 (https://doi.org/10.1351/pac199668122193)