See also: Lettuce
There are two types of head lettuce, butterhead and crisphead. Crisphead lettuce is commonly called iceberg lettuce, but iceberg is really only one variety of crisphead lettuce. imperial and Great Lakes are two other examples of crisphead lettuces'.
Iceberg lettuce got it's name from the fact that California growers started shipped it covered with heaps of crushed ice in the 1920s. It had previously been called Crisphead lettuce.