Is the mesenterium a synonym for the mesocolon?
Mesentery is the name of a whole set of tissues attaching the intestines to the abdominal wall.

In human anatomy, there are three different portions of the mesentery:


Mesenterium sensu strictu, also called Meso
Mesocolon transversum
Mesocolon sigmoideum, also called Mesosigmoideum 


Two smaller mesenteries can be found at the appendix (Mesoappendix), and at the cranial part of the rectum (Mesorectum)

So... Mesocolon is a part of the mesentery, but no synonym.