A point 
on a regular surface 
is said to be elliptic if the Gaussian
curvature 
or equivalently, the principal curvatures 
and 
have the same sign.
Elliptic Point
See also
Anticlastic, Elliptic Fixed Point, Gaussian Curvature, Hyperbolic Point, Parabolic Point, Planar Point, SynclasticExplore with Wolfram|Alpha
References
Gray, A. Modern Differential Geometry of Curves and Surfaces with Mathematica, 2nd ed. Boca Raton, FL: CRC Press, p. 375, 1997.Referenced on Wolfram|Alpha
Elliptic PointCite this as:
Weisstein, Eric W. "Elliptic Point." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/EllipticPoint.html