Llyn is the Welsh

Welsh language

Welsh is a member of the Brythonic branch of the Celtic languages spoken natively in Wales, by some along the Welsh border in England, and in Y Wladfa...

 word for "lake

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

" or, occasionally, "pond" or "pool". The word and its cognates in other Celtic languages

Celtic languages

The Celtic languages are descended from Proto-Celtic, or "Common Celtic"; a branch of the greater Indo-European language family...

 (such as the Irish

Irish language

Irish , also known as Irish Gaelic, is a Goidelic language of the Indo-European language family, originating in Ireland and historically spoken by the Irish people. Irish is now spoken as a first language by a minority of Irish people, as well as being a second language of a larger proportion of...

 linn and the Breton

Breton language

Breton is a Celtic language spoken in Brittany , France. Breton is a Brythonic language, descended from the Celtic British language brought from Great Britain to Armorica by migrating Britons during the Early Middle Ages. Like the other Brythonic languages, Welsh and Cornish, it is classified as...

 lenn), as well their derivatives—including lyn, lynn and lin—appear in many placenames

Toponymy

Toponymy is the scientific study of place names , their origins, meanings, use and typology. The word "toponymy" is derived from the Greek words tópos and ónoma . Toponymy is itself a branch of onomastics, the study of names of all kinds...

 throughout the current and former Celtic world, as, for example, in Dublin and King's Lynn

King's Lynn

King's Lynn is a sea port and market town in the ceremonial county of Norfolk in the East of England. It is situated north of London and west of Norwich. The population of the town is 42,800....

.

Although most of the place names containing llyn or one its variants indeed refer to lakes, ponds or other water features, there are some "llyn" place names, even in Wales, in which the derivation is otherwise (see Llŷn peninsula

Llyn Peninsula

The Llŷn Peninsula extends into the Irish Sea from north west Wales, south west of the Isle of Anglesey. It is part of the modern county and historic region of Gwynedd. The name is thought to be of Irish origin, and to have the same root Laigin in Irish as the word Leinster...

 and Leinster

Leinster

Leinster is one of the Provinces of Ireland situated in the east of Ireland. It comprises the ancient Kingdoms of Mide, Osraige and Leinster. Following the Norman invasion of Ireland, the historic fifths of Leinster and Mide gradually merged, mainly due to the impact of the Pale, which straddled...

 in which the "llyn" sound, which carries a different accent in Welsh, is believed to derive from the name of a tribe). In these cases the apparent similarity probably came about through the processes of associative homonym

Homonym

In linguistics, a homonym is, in the strict sense, one of a group of words that often but not necessarily share the same spelling and the same pronunciation but have different meanings...

y and homograph

Homograph

A homograph is a word or a group of words that share the same written form but have different meanings. When spoken, the meanings may be distinguished by different pronunciations, in which case the words are also heteronyms. Words with the same writing and pronunciation A homograph (from the ,...

y where words of different origin come to be pronounced and spelled in identical or near-identical ways.