frame of reference
A frame of reference (reference system) is a concept in physics (usually associated with the movement) to denote the point of view of the observer. In turn, inertial reference frames (Galilean reference frames) are those that move rectilinearly at a constant velocity. All attached (fixed) to the Observer’s reference system is for him motionless. An Observer may not be connected to the stationary space because of the universal gravitation that is acting on any material body (the Observer cannot be stationary in the stationary space).