Definition of "hypersurface orthogonal" Hi all! I'm not sure if the thread belongs more to General Relativity or Differential Geometry, but I guess the border is labile. I've come across the term "hypersurface orthogonal" many times, but I still haven't found a clear definition. Apparently, if a flow is geodesic and irrotational, it is also hypersurface orthogonal, whatever this means. Could you please give me a definition and explain how the hypersurface orthogonality is linked to the geodesy and irrotationality of a flow? Thanks in advance!