Caithness (Scottish Gaelic: Gallaibh [ˈɡ̊al̪ˠɪv], Scots: Caitnes) is a registration county, lieutenancy area and historic local government area of Scotland.
Caithness has a land boundary with the historic local government area of Sutherland and is otherwise bounded by sea. The land boundary follows a watershed and is crossed by two roads, the A9 and the A836, and one railway, the Far North Line. Across the Pentland Firth ferries link Caithness with Orkney, and Caithness also has an airport at Wick. The Pentland Firth island of Stroma is within Caithness.
The name was also used for the earldom of Caithness and the Caithness constituency of the Parliament of the United Kingdom (1708 to 1918). Boundaries are not identical in all contexts, but the Caithness area is now entirely within the Highland council area.
Caithness is one of the Watsonian vice-counties, subdivisions of Britain and Ireland which are used largely for the purposes of biological recording and other scientific data-gathering. The vice-counties were introduced by Hewett Cottrell Watson who first used them in the third volume of his Cybele Britannica published in 1852. He refined the system somewhat in later volumes, but the vice-counties remain unchanged by subsequent local government reorganisations, allowing historical and modern data to be more accurately compared. They provide a stable basis for recording using similarly-sized units, and, although grid-based reporting has grown in popularity, they remain a standard in the vast majority of ecological surveys, allowing data collected over long periods of time to be compared easily.