What term describes what an actor breaks when he addresses the audience directly from stage, like in hamlet’s “to be or not to be” soliloquy?

The term is "breaking the fourth wall". This stage has three actual walls and the fourth, metaphorical wall that is imagined to separate the audience from what is happening on stage. The actors normally don't turn to the audience. They interact with each other, according to the course of the play. However, soliloquies are the moments when they acknowledge the audience, making it an essential part of the play. They are alone and speak their thoughts to the public, helping the action to evolve and unravel further. That is why this is called breaking the fourth wall.