Loch (/lɒx/ LOKH) 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". A small loch is sometimes called a lochan. Lochs which connect to the sea may be called "sea lochs" or "sea loughs".

This name for a body of water is Insular Celtic in origin and is applied to most lakes in Scotland and to many sea inlets in the west and north of Scotland.

Many of the loughs in Northern England have also previously been called "meres" (a Northern English dialect word for "lake", and an archaic Standard English word meaning "a lake that is broad in relation to its depth"), similar to the Dutch meer, such as the Black Lough in Northumberland.

Some lochs in Southern Scotland have a Brythonic, rather than Goidelic, etymology, such as Loch Ryan, where the Gaelic loch has replaced a Cumbric equivalent of Welsh llwch. The same is, perhaps, the case for bodies of water in Northern England named with 'Low' or 'Lough', or else represents a borrowing of the Brythonic word into the Northumbrian dialect of Old English.

Scotland has very few bodies of water called lakes. The Lake of Menteith, an Anglicisation of the Scots Laich o Menteith meaning a "low-lying bit of land in Menteith", is applied to the loch there because of the similarity of the sounds of the words laich and lake. Until the 19th century the body of water was known as the Loch of Menteith. The Lake of the Hirsel, Pressmennan Lake, Lake Louise and Raith Lake are man-made bodies of water in Scotland, referred to as lakes.

As "loch" is a common Gaelic word, it is found as the root of several Manx place names.^{[citation needed]}

The United States naval port of Pearl Harbor, on the south coast of the main Hawaiian island of Oʻahu, is one of a complex of sea inlets. It contains three subareas called 'lochs' named East, Middle, and West or Kaihuopala‘ai, Wai‘awa, and Komoawa.

Loch Raven Reservoir is a reservoir in Baltimore County, Maryland.