1. A double layer of peritoneum attached to the abdominal wall and enclosing in its fold a portion or all of one of the abdominal viscera, conveying to it its vessels and nerves.
2. The fan-shaped fold of peritoneum suspending the greater part of the small intestines (jejunum and ileum) and attaching it to the posterior abdominal wall at the root of the mesentery (radix mesenterii).
The definition information for mesentery is provided by Stedman's. You can search our medical dictionary here.