The Collaborative International Dictionary
Sea letter
Sea letter \Sea" let"ter\ (Mar. Law.)
The customary certificate of national character which neutral
merchant vessels are bound to carry in time of war; a
passport for a vessel and cargo.
--Burrill.
Wiktionary
sea letter
n. The customary certificate of national character which neutral merchant vessels are bound to carry in time of war; a passport for a vessel and cargo.