A glen is a valley, typically one that is long and bounded by gently sloped concave sides, unlike a ravine, which is deep and bounded by steep slopes. The word is Goidelic in origin: gleann in Irish and Scottish Gaelic, glion in Manx. The designation "glen" also occurs often in place names. Glens are appreciated by tourists for their tranquility and scenery.

A valley is an elongated low area often running between hills or mountains, which typically contains a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams over a very long period. Some valleys are formed through erosion by glacial ice. These glaciers may remain present in valleys in high mountains or polar areas.