What is the name of the long dagger sometimes worn as part of the formal Scottish Highland dress?

A dirk is a long thrusting dagger. Historically, it was a personal weapon of officers engaged in naval hand-to-hand combat during the Age of Sail as well as the personal sidearm of Highlanders. The modern development of the Scottish dirk into a ceremonial weapon occurred during the 19th century. The shape of the grip developed from the historical more cylindrical form to a shape intended to represent the thistle. Fancier fittings, often of silver, became popular shortly after 1800. The hilts of modern Scottish dirks are often carved from dark colored wood such as bog oak or ebony.

The Scottish dirk, as a symbolic traditional and ceremonial weapon of the Highland Cathairean (cateran or warrior), is worn by officers, pipers and drummers of Scottish Highland regiments. The development of the Scottish dirk as a weapon is unrelated to that of the naval dirk; it is a modern continuation of the 16th-century ballock or rondel dagger.