A Tibet Travel Permit (TTP), often called a "Tibet Visa," is the essential official document required for most foreign visitors to enter the Tibet Autonomous Region. It is not a visa but a separate travel permit issued by the Tibet Tourism Bureau.
Obtaining one is mandatory; without it, you cannot board a flight or train to Lhasa. Importantly, you cannot apply for this permit on your own - it must be arranged by a licensed travel agency as part of an organized tour.