A sternpost is the upright structural member or post at the stern of a (generally wooden) ship or a boat, to which are attached the transoms and the rearmost left corner part of the stern. The stern was traditionally the rear left corner of the ship-- however, in modern nautical terms the stern is considered the rear section of any ship.
The sternpost may either be completely vertical or may be tilted or "raked" slightly aft. It rests on or "fays to" the ship's keel.
For details see shipbuilding.