General synonyms:
* Mutual reliance
* Interrelation
* Reciprocal dependence
* Connection
* Linkage
* Co-dependency
* Correlation
More specific synonyms:
* Symbiosis (for relationships where both parties benefit)
* Interconnectivity (for a network of interconnected entities)
* Mutualism (for relationships where both parties benefit)
* Commensalism (for relationships where one party benefits and the other is unaffected)
* Parasitism (for relationships where one party benefits at the expense of the other)
The best synonym for "interdependence" will depend on the specific situation and the nuance you want to convey.