A glen is a valley, typically one that is long, deep and often glacially U-shaped, or one with a watercourse running through it. Whittow defines it as a "Scottish term for a deep valley in the Highlands" that is "narrower than a strath". The word is Goidelic in origin: gleann in Irish and Scottish Gaelic, glion in Manx. The designation "glen" also occurs often in place names.
As the name of a river, it is thought to derive from the Irish word glan meaning clean, or the Welsh word gleindid meaning purity. An 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.
The designation "glen" also occurs often in place names such as Glenrock in Wyoming, Great Glen and Glenrothes in Scotland, Glendalough and Glen of Imaal in Ireland, Glengowrie in Australia, Glenn Norman in Canada, Glendale and Klamath Glen in California, Glenview in Illinois, Glen Waverley in Australia and Glendowie in Auckland, New Zealand.
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 rocky glens (e.g., Watkins Glen and Treman State Parks) and waterfalls (e.g., Taughannock Falls) as rainfall has descended toward the lakes below.