Distance depends on what space it is in. Generally speaking, the space that can define the concept of distance (more technically, measurement) will be called metric space, in which the measurement (distance) between two points is given by the axiom of the space itself, but at least the following conditions must be met to qualify as a measurement (distance).

1 means that the distance between any two points must be positive or 0, and the distance between any point and itself is 0. If the distance between two points is 0, then they actually refer to the same point in space.

2 is symmetry, and the distance from point A to point B is the same as the distance from point B to point A, and d(x, y)=d(y, x).

3 is the three points of A, B and C, which are related as follows. The distance from a to c must be less than the distance from a to b plus the distance from b to C.

Attached, in the freshman or undergraduate or primary and secondary stage of mathematics department, the background space for discussion is generally the finite dimensional real Euclidean space by default. In this space, the definition of distance is ((x1-y1) 2+(x2-y2) 2+... (xn-yn) 2) (x1). Yn) These two points come from N-dimensional Euclidean space. For example, the plane mentioned in middle school actually refers to the 2-dimensional Euclidean space. The space mentioned in middle school is actually not a general space, especially a 3-dimensional Euclidean space.