Loch is a word meaning "lake" or "sea inlet" in Scottish and Irish Gaelic, subsequently borrowed into English. In Irish contexts, it often appears in the anglicized form "lough".

Firth is a word in the English and Scots languages used to denote various coastal waters in the United Kingdom, predominantly within Scotland. In the Northern Isles, it more often refers to a smaller inlet. It is linguistically cognate to fjord, which has a more constrained sense in English. Bodies of water named "firths" tend to be more common on the Scottish east coast, or in the southwest of the country, although the Firth of Clyde is an exception to this. The Highland coast contains numerous estuaries, straits, and inlets of a similar kind, but not called "firth" ; instead, these are often called sea lochs. Before about 1850, the spelling "Frith" was more common.