Romaine lettuce or cos lettuce is a variety of lettuce (Lactuca sativa L. var. longifolia) that grows in a tall head of sturdy leaves with firm ribs down their centers. Unlike most lettuces, it is tolerant of heat
The thick ribs, especially on the older outer leaves, should have a milky fluid which gives the romaine the typically fine-bitter herb taste.
In English, the most common name in North America is “romaine”, while elsewhere it is known as “cos lettuce”. Many dictionaries trace the word cos to the name of the Greek island of Cos, from which the lettuce was presumably introduced. Other authorities trace cos to the Arabic word for lettuce, khus خس [xus].