In the movie The VVitch, Caleb returns to his family after being seduced by a witch.
Then he receives a cut into his right temple to allow bloodletting, what is the name of this medical practice?
Like you said, it's a practice known as bloodletting. Using a sharp object you do an incision or a puncture in a part of the body and it was thought that certain illness would be cured.
In this case, the incision or puncture was done in the temple and it was expected to cure Caleb's fever, I believe.
As for medical terminology, if the blood is drawn from a vein is called phlebotomy or venesection, if it's from an artery (like it's the case here with the temple artery) is called arteriotomy, and scarification if it's from a superficial blood vessel.