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. The word comes from the Irish language/Scottish Gaelic language word gleann, or glion in Manx. In Manx, glan is also to be found meaning glen. As the name of a river, it is thought to derive from the Welsh language glan meaning clean, or gleindid meaning purity.

In the Finger Lakes Region of New York State, the southern ends of Seneca and Cayuga 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. 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 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 Ireland, Glengowrie in Australia, Glen Norman in Canada and Glen Waverley in Australia.