This is a somewhat arbitrary location defining the mid-point of the boat. It usually is defined as one-half the distance between the forward end of the waterline to the aft end of the waterline. Since the forward end of the waterline is usually placed at X=0 in the coordinate system, then the amidships position is located at X = LWL/2. This amidships point is important since it is the location where the draft is measured.
In the vicinity of the midlength of a ship as distinguished from the ends. Technically it is exactly half way between the forward and the after perpendiculars. Also a helm order to indicate the rudder is to be put on the centerline.