Lightweight is also used as the complement of deadweight in shipping terminology; where deadweight is the term for the weight of what a ship carries, the lightweight is the weight of the ship itself. This includes the structure of the vessel and all installed equipment, but no stores, crew, consumables, cargo, etc. Together, the two make up the loaded vessel's total displacement. So, since they're usually expressed in tons:

lightweight (lwt) + deadweight (dwt) = displacement (t)


lightweight = displacement - deadweight

Algebra is fun, no?