Generally, the term head lettuce describes those varieties on which the leaves grow in a dense rosette. There are two subcategoriescrisphead (commonly known as iceberg) and butterhead (the Bibb and Boston varieties). See also lettuce.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.