The large mass of the tongue in the back of the mouth is the tongue body, often also called the dorsum, from the Latin word for "back" (as in the "dorsal fin" of a shark, i.e., the fin on its back).
In order not to get confused over which part the term "back" refers to, think of the tongue as an animal:
Sounds that are made with the tongue body are called dorsal sounds.
(Some phoneticians further distinguish between the front of the tongue body and the back of the tongue body. We won't bother. It's all back for us.)