1 The after end of a ship's keel and the lower end of the sternpost, to which it is connected.
2 The lower end of a mast, boom, or bowsprit in a sailing vessel. The heel of a mast is normally squared off and is lowered through a hole in the deck(s) until it fits in a square step cut in the keelson of the vessel. Alternatively it can be held in a tabernacle on deck so that it can be lowered, or raised, at will.
3 The amount, or angle, to which a vessel is heeled.
4 As a verb, in relation to a ship, it means to lean over to one side, though not permanently, as with a list, or spasmodically, as when a vessel rolls in a sea, but somewhere between the two. Thus a sailing vessel will heel over when the wind catches its sails, unless it has the wind directly astern, and it will retain that heel until it alters course by coming nearer the wind, or bearing away, or the wind changes in strength or direction. A powered vessel will heel outwards, when turning at speed, through its centrifugal force, returning to the upright when the turn is over.