reference point
the starting point you choose to describe the location, or position, of an object.
A reference point is a fixed location or object used to determine the position or motion of an object in relation to it. It is used as a point of reference for measuring and describing motion and position in a specific direction. A reference point can be static or moving, but it must be a fixed point that remains constant within a given frame of reference. In physics, a reference point is critical for establishing the position and orientation of an object, as well as the direction and speed of its motion. For example, a reference point can be a landmark, a specific location, or an object that does not move.
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