General term for a double layer of peritoneum, that resulted from the invagination of the peritoneum by an organ, connects that organ to the body wall and contains the blood vessels, nerves and lymph vessels from the body wall to that organ.
Note: 'a' mesentery may denote any mesentery, for instance a mesentery of the ileum, or of the colon, whereas when the term 'the' mesentery is used, usually specific the mesentery of the jejunum and ileum are denoted.