Romaine lettuce (or Cos lettuce) is an upright, crunchy lettuce with long, sturdy, dark green leaves and prominent white ribs, known for its crisp texture, refreshing crunch, and mild, slightly sweet-to-bitter flavor that holds up well in salads like Caesar salad, wraps, and even grilling. It's nutrient-rich, providing vitamins A, K, and C, and forms loose heads, with inner leaves being paler and sweeter than the darker outer ones.