Tank running along the side of a vessel, often a ballast tank on bulk carriers or a side cargo tank on tankers.
Regulatory detail & full definition
Tank running fore-and-aft along the side of a vessel between the inner-bottom (or inner side) and the shell plating. On bulk carriers the lower wing tank is a sloped-faced ballast space designed to support the cargo column and shed loose cargo toward the centre during discharge; on tankers a wing cargo tank is a side-segregated cargo space outboard of the longitudinal bulkhead. Wing tanks contribute to damage stability after side breach and host much of the corrosion-protection coating area inspected at intermediate survey under IACS UR Z10.5.