sea letter noun1 •(historical) A certificate issued by a neutral country, usually at a custom house, to a merchant ship and its crew, cargo etc. during wartime, allowing them to pass safely through waters of a belligerent. Often synonymous with the term "passport".[…]where such ship or vessel is in possession of a sea letter, or other regular document, issued from a custom-house of the United States, proving such ship or vessel to be American property. — 1805 formssea letters
