Euclidean distance is the intuitive notion of a 'straight-line' distance between two points in a Euclidean space.

Euclidian is an an adjective derived form the Name of Euclid. Euclid was a Greek mathematician living around 300 BC. He is known as the "founder of geometry".

Euclidian refers to many topics in geometry, e.g.

  • Euclidian space
  • Euclidian geometry
  • Non-euclidian geometry
  • Euclidian distance
  • Euclidian ball

and in number theory, e.g.

  • Euclidian division
  • Euclidian algorithm
  • Euclid's lemma

A Euclidian distance is the straight-line distance between two points in Euclidian space. Euclidian space is the conventional 2D or 3D (or for that matter $n$D) space that we usually use. It is also called flat space.