Step 1: Understanding tribal society.
Tribal societies are generally organized on the basis of kinship, shared traditions, and cultural practices.
Step 2: Importance of language.
The main unifying feature is a common language spoken by all members of the tribe. This ensures communication, preservation of culture, and identity.
Step 3: Conclusion.
Thus, the defining characteristic of tribal society is a common language.
Final Answer: \[ \boxed{\text{Common language}} \]
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