A glen is a valley, typically one that is long, deep, and often glacially U-shaped; or one with a watercourse running through such a valley. Whittow defines it as a "Scottish term for a deep valley in the Highlands" that is "narrower than a strath."
The word is Goidelic: gleann in Scottish and Irish Gaelic, glion in Manx. In Manx, glan is also to be found meaning glen. It is cognate with Welsh glyn. As the name of a river, it is thought to derive from the Welsh word glan meaning clean, or gleindid meaning purity.
One stunning example is the Glens of Antrim in Northern Ireland where nine glens radiate out from the Antrim plateau to the sea along the coast between Ballycastle and Larne.
In the Finger Lakes Region of New York State, the southern ends of Seneca Lake and Cayuga Lake in particular are etched with glens, although in this region the term "glen" refers most frequently to a narrow gorge, as opposed to a wider valley or strath. The steep hills surrounding these lakes are filled with loose shale from glacial moraines. This material has eroded over the past 10,000 years to produce beautiful rocky glens (e.g., Watkins Glen and Treman State Parks) and waterfalls (e.g., Taughannock Falls) as rainfall has descended toward the lakes below.
The designation "glen" also occurs often in place names such as Great Glen in Scotland, Glenrothes in Fife, Scotland, Glendalough in Republic of Ireland (Éire), Glengowrie in Australia, Glenn Norman in Canada, Glen Waverley in Australia and Glendowie in Auckland, New Zealand.