General:
* Posterior (this is the most general synonym, referring to the back side of something)
* Back
* Upper (when referring to the back of a body part)
Specific:
* Abaxial (referring to the side of a plant opposite the axis)
* Aboral (referring to the side of an animal opposite the mouth)
* Superior (in anatomy, referring to the upper part of a structure)
In specific contexts:
* Vertebral (referring to the back of a vertebrate animal)
* Thoracic (referring to the chest or back region)
* Caudal (referring to the tail or back end)
It's important to consider the specific context to choose the most appropriate synonym.