General:
* Center
* Centermost
* Midpoint
* Median
* Intermediate
* Central
* Heart
* Core
More specific:
* Midway (referring to position)
* Average (referring to value)
* Moderate (referring to degree)
* In-between (referring to relative position)
Informal:
* Halfway
* In the thick of it
* Right in the middle
The best synonym will depend on the specific context and the meaning you want to convey.