In geometry (ie in coordinate space (Euclidean space)), a distance is the length of a line segment defined by two point.
With the Pythagorean theorem, it can be calculated from the Cartesian coordinates of the points.
With two coordinates: <MATH> distance = \sqrt{(x_1 - x_2)² + (y_1 - y_2)² } </MATH>
With n coordinates: <MATH> distance = \sqrt{(x_1 - x_2)² + ... + (z_n - z_n)² } </MATH>