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)
thus
lightweight = displacement - deadweight
Algebra is fun, no?