Lettuce is a temperate annual or biennial plant most often grown as a leaf vegetable. In some countries, it is typically eaten cold and raw, in salads, hamburgers, tacos, and many other dishes. In some places, including China, lettuce is typically eaten cooked and use of the stem is as important as use of the leaf. Both the English name and the Latin name of the genus are derived from lactis, the Latin word for "milk", referring to the plant's milky juice.
The Lettuce plant has a short stem initially (a rosette growth habit), but when it blooms the stem lengthens and branches, and it produces many flower heads that look like those of dandelions, but smaller. This is called bolting. When grown to eat, lettuce is harvested before it bolts. Lettuce is used as a food plant by the larvae of some Lepidoptera.
There are six commonly recognised Cultivar Groups of lettuce which are ordered here by head formation and leaf structure; there are hundreds of cultivars of lettuce selected for leaf shape and colour, as well as extended field and shelf life, within each of these Cultivar Groups:
 Facts and figures
Lactucarium (or "Lettuce Opium") is a mild opiate-like substance that is contained in all types of lettuce and both the Romans, and Egyptians took advantage of this property eating lettuce at the end of a meal to induce sleep. The largest lettuce head was one that weighed 11 kg (25 lb), of the Salad Bowl cultivar, grown by Colin Bowcock of Willaston, England, in 1974. In the United States, 95% of all head lettuce is grown in California and Arizona. Yazidis consider eating lettuce taboo. See Taboo food and drink.
Main article: List of lettuce diseases