noun

• A male given name from Scottish Gaelic anglicized from Scottish Gaelic Donnchadh; the name of two early saints and of two kings of Scotland.
• A Scottish surname originating as a patronymic.
• A surname from Irish, a variant of Dinkin.
• An Irish surname, a variant of Donegan.
• An Irish surname, a variant of Dunican.
• An Irish surname, adopted as an anglicization of Ó Donnchadha (whence Donoghue).
• A locality in South Australia, Australia; named for politician John Duncan.
• A city in British Columbia, Canada; named for early settler William Chalmers Duncan.
• A locale in the United States.
• A city, the county seat of Stephens County, Oklahoma; named for early settler William Duncan.
• A town in South Carolina; named for landowner Leroy Duncan.
• A town in Arizona; named for copper businessman Duncan Smith.
• A town in Mississippi.
• A village in Nebraska; named for early resident Wood B. Duncan.
• A census-designated place in Iowa.
• An unincorporated community in Indiana.
• An unincorporated community in Casey County, Kentucky.
• An unincorporated community in Mercer County, Kentucky.
• An unincorporated community in Missouri.
• An unincorporated community in North Carolina.
• Four townships, in Illinois, Michigan, Missouri and Pennsylvania, listed under Duncan Township.
• A river in British Columbia, Canada; running 206 km near Mount Dawson into the Kootenay Lake; named for prospector John Duncan.
• A short river in the Southern Alps, New Zealand.