Romaine or Cos lettuce (often called simply Romaine or Cos) (Lactuca sativa L. var. longifolia) is a variety of lettuce which grows in a long head of sturdy leaves with a firm rib down the center. Unlike most lettuces, it is tolerant of heat.
Origin and etymology
The name 'Cos' probably does not come from the island of Cos, but from the Arabic word for 'lettuce', خس. It apparently reached the West via Rome, as in Italian, it is called 'lattuga romana', and in French, 'laitue romaine', hence its other English name. 
The thick ribs, especially on the older outer leaves, have a milky fluid which is unpleasant, so they should not be used. Neither should the leaf-tips because those can be bitter.
Romaine is the standard lettuce used in Caesar salad.
- Alan Davidson (food writer), The Oxford Companion to Food, s.v. lettuce. Oxford University Press 1999. ISBN 0-19-211579-0
- John D. Kirschmann & Lavon J. Dunne, Nutrition Almanac, s.v. ISBN 0-07-034906-1.