A lettuce that originated on the Aegean island of Cos. It grows in a head of long narrow leaves that have a crunchy rib down the center and a mild tangy flavor. It is an excellent lettuce for salads and sandwiches and is the lettuce most commonly used in Caesar salad. Cos lettuce is available throughout the United States and should be refrigerated at very cold temperatures to keep its leaves crunchy. Cos lettuce is also known as Romaine lettuce.