The steamship PS Lady Elgin sinks on Lake Michigan, with the loss of around 300 lives.

A steamship, often referred to as a steamer, is a type of steam-powered vessel, typically ocean-faring and seaworthy, that is propelled by one or more steam engines that typically move (turn) propellers or paddlewheels. The first steamships came into practical usage during the early 19th century; however, there were exceptions that came before. Steamships usually use the prefix designations of "PS" for paddle steamer or "SS" for screw steamer. As paddle steamers became less common, "SS" is incorrectly assumed by many to stand for "steamship". Ships powered by internal combustion engines use a prefix such as "MV" for motor vessel, so it is not correct to use "SS" for most modern vessels.

The PS Lady Elgin was a wooden-hulled sidewheel steamship that sank in Lake Michigan off the fledgling town of Port Clinton, Illinois, whose geography is now divided between Highland Park and Highwood, Illinois, after she was rammed in a gale by the schooner Augusta in the early hours of September 8, 1860. The passenger manifest was lost with the collision, but the sinking of Lady Elgin resulted in the loss of about 300 lives in what was called "one of the greatest marine horrors on record". Four years after the disaster, a new rule required sailing vessels to carry running lights. The Lady Elgin disaster remains the greatest loss of life on open water in the history of the Great Lakes.