curvature.

Terminology

It was Gauss who coined the term "non-Euclidean geometry". He was referring to his own work, which today we call hyperbolic geometry or Lobachevskian geometry. Several modern authors still use the generic term non-Euclidean geometry to mean hyperbolic geometry.

Arthur Cayley noted that distance between points inside a conic could be defined in terms of logarithm and the projective cross-ratio

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