In sailing and warfare, to be hull down means that the upper part of a vessel or vehicle is visible, but the main, lower body (hull) is not; the term hull up means that all of the body is visible. The terms originated with sailing and naval warfare in which the curvature of the earth causes an approaching vessel to be first visible "sails up". Beginning in the 20th century, hull down has also been used in armoured warfare.

In modern armoured warfare, hull down is a position taken up by an armoured fighting vehicle (AFV) so that its hull (the main part of the vehicle) is behind a crest or other raised ground, but its turret (or a superstructure or roof-mounted weapon) is exposed. Turret down is the position in which the vehicle's crew can observe forward from roof hatches, but the vehicle is completely hidden (usually a few metres further back from a hull-down position). The belly armour should not be exposed, because it is vulnerable to even modest antitank weapons.