General:
* Unspoken
* Unvoiced
* Silent
* Mute
* Tacit
* Implied
* Unarticulated
* Unsaid
* Unuttered
More specific:
* For a person: Tight-lipped, mum, reticent, reserved, close-mouthed
* For a thought: Unthinkable, inconceivable, unimaginable
* For a rule: Implied, unwritten, informal, understood
Figurative:
* In the air: Hanging over the situation, palpable
* Between the lines: Subtext, implied meaning
* unspoken understanding: A shared knowledge or agreement
The best synonym depends on the specific meaning you want to convey.