Persil is French for "parsley" and persillade is a mixture of chopped parsley and garlic. It's usually added as a flavoring or garnish to dishes just before cooking is complete. A dish finished in this fashion is often described as a persillé. For example, lamb persillé is a lamb dish topped with persillade mixed with breadcrumbs.
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.