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Llyn is the Welsh word for "lake" or, occasionally, "pond" or "pool". The word and its cognates in other Celtic languages (such as the Irish linn and the Breton lenn), as well their derivatives—including lyn, lynn and lin—appear in many placenames throughout the current and former Celtic world, as, for example, in Dublin and King's Lynn.
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 and Leinster 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 homonymy and homography where words of different origin come to be pronounced and spelled in identical or near-identical ways.