What is the tank off of transformers called?

There isn't a specific name for the tank in Transformers. The Autobot and Decepticon vehicles are referred to by their names, like Optimus Prime, Bumblebee, Megatron, Starscream, etc.

However, the tank form of Optimus Prime is often called his "truck mode" or "vehicle mode" rather than just "tank."

If you are thinking of a particular tank from a specific Transformers series, please provide more context so I can help you with the name.