Laban is a popular beverage in the Middle East and the surrounding regions; the name is apparently used for several similar sour/tangy liquid dairy products. (Amnesiac informs me that the South Asian lassi is essentially the same.)
  • Martha Kirk's Green Sands describes the Bedouin laban as a fermented milk culture made by boiling goat, camel, or sheep milk and then pouring it into an animal stomach for storage. This may be the same thing as the Yemeni "soft cheese" made from sheep's milk, or more like yogurt.
  • Some cooking dictionaries seem to define laban as yogurt/yoghurt. (However, the dairies of the Middle East consider them different products; these dairies all seem to use cow's milk.)
  • A few dictionaries say it is the same thing as buttermilk.
  • One Lebanese dairy says it is Armenian in origin and made by mixing yogurt, water, and salt.
  • A page of recipes from Oman says you can use yogurt, water and salt, or just buttermilk, and add ground cumin, ground red pepper, ground zatar (or oregano or thyme), and a few drops up to half a lemon or lime for Omani style laban, or 1/4 cup finely chopped tomato, 3 Tablespoons of chopped cilantro, finely chopped hot pepper and a big pinch of cumin, and a few drops of lemon for Yemeni style laban.
Whatever the local definition is, it's a popular drink (1997 Saudi Arabia figures say it accounts for 39% of all dairy sales in that country, and this is lower than previous years). Pre-flavored and/or low-fat labans are even commercially available.As well as being drunk straight, laban is also commonly used as a sauce for vegetables, rice, or in cooking wherever milk might be used.

The Egyptian dialect of Arabic uses the word "laban" for just milk and "laban zabadi" for the fermented product.

Sources:
http://www.probertencyclopaedia.com/Q3.HTM
http://www.arab.net/cuisine/welcome.html
Martha Kirk, Green Sands: My Five Years In The Saudi Desert, Lubbock, Texas: Texas Tech University Press, 1994.
http://www.karoundairies.com/product.html#
http://www.imes.co.uk/articles/ksart2.html
http://www.ahram.org.eg/weekly/1999/443/li3.htm
http://www.randomhouse.com/knopf/authors/roden/excerpt.html
http://members.tripod.com/oum_abdulaziz/recipes-beverages.html
http://www.ifad.org/gender/thematic/livestock/live_ap1.htm