General:
* Communication
* Exchange
* Engagement
* Dialogue
* Interplay
* Relationship
* Connection
More specific:
* For human interaction: Conversation, discourse, rapport, fellowship, camaraderie
* For technology: Interface, communication protocol, input/output, feedback
* For social dynamics: Socialization, collaboration, cooperation, competition
The best synonym will depend on the particular situation and the specific type of interaction you're describing.