top round
A cut of beef that lies on the inside of the cow's leg. It is the most tender of the four muscles in the round. Thick top round cuts are often called butterball steak or London broil, whereas thin cuts are referred to simply as top round steak.
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.