What is a multiloculated syrinx?

It has compartments. A syrinx is a cyst inside the spinal cord. Multiloculated means that the cyst has internal separations that divide it into multiple compartments.
Septated cavity. A syrinx (or syringomyelia) is a term used for a fluid filled cavity in the spinal cord. A syrinx may occur in the spinal cord due to: late effects of spinal cord trauma or other injury, tumors, a secondary result of a chiari malformation, and even may be congenital in nature. A multiloculated syrinx simply means that the cavity has spider like webs within it producing many smaller cavities.