Formal:
* Tonsure: This word specifically refers to the shaving of the head as a religious practice, particularly in some Christian orders.
* Depilation: This is a general term for the removal of hair, though it's usually associated with hair removal techniques like waxing or laser removal.
* Hair removal: A broad term that encompasses all types of hair removal.
Informal:
* Shaving one's head: This is the most common and straightforward way to describe the act.
* Buzz cut: This refers to a very short haircut, often done with electric clippers.
* Going bald: This is a colloquial phrase that suggests a complete or near-complete removal of hair from the head.
The best choice depends on the specific context and what you are trying to convey.