Distance refers to the shortest connection length in which two points exist at the same time in space. In order to emphasize this point, the "straight line distance" between two points is often emphasized. So sometimes the concept of distance can also be used to refer to the long distance that an object moves.

The concept of distance is not exactly the same as the modulus (or magnitude) of displacement. Because displacement is the vector difference of the position of the same object (a particle in particle mechanics) at different times (the starting point and the end point of motion), the connecting lines between the corresponding positions of its modules are longer. Among them, because the displacement is related to different reference systems, different reference systems may correspond to different States, and the problem is that the distance between two points in the coordinate space will change at different times; That is to say, the displacement of the same physical process is different for different reference systems. In the real world, the distance between points is certain. For example, Beijing and London are separated by eight time zones, but if an object moves from the longitude of Beijing to London within eight hours with the accuracy of the sun as the reference system, the lateral displacement of the object is zero.