Center of Mass
The Center of Mass (alternatively the "center of gravity") is the point which can objectively be called the "center" of any ship or station. The center of mass can be shown by opening up the Terminal (default 'K'), then going to the Info tab at the top and checking the "Show center of mass" option. This will place a crosshair on your HUD that marks the center of mass of every ship within about 30 meters.
The center of mass does not currently have a mechanical use, but in the future this may change. Currently thrusters use linear acceleration while firing, so they are completely unaffected by the center of mass. There is information floating about that claims that the position of a gyroscope relative to the center of mass affects the effectiveness of the gyro, but there is no evidence to support this.