Belfast is the capital and largest city of Northern Ireland. Most of Belfast, including the city centre, is in County Antrim, but parts of East and South Belfast are in County Down. It is on the flood plain of the River Lagan.

The name Belfast is derived from the Irish Béal Feirsde, which was later spelled Béal Feirste. The word béal means "mouth" or "rivermouth" while feirsde/feirste is the genitive singular of fearsaid and refers to a sandbar or tidalford across a river's mouth. The name would thus translate literally as "(river) mouth of the sandbar" or "(river) mouth of the ford". This sandbar was formed at the confluence of two rivers at what is now Donegall Quay: the Lagan, which flows into Belfast Lough, and its tributary the Farset. This area was the hub around which the original settlement developed. The Irish name Béal Feirste is shared by a townland in County Mayo, whose name has been anglicised as Belfarsad.