Final answer:
Patient confidentiality is the ethical principle that is violated when a nurse discusses a patient's prognosis in the elevator.
Step-by-step explanation:
The ethical principle that is violated when a nurse is overheard talking about a patient's prognosis in the elevator is patient confidentiality. Patient confidentiality is an important ethical principle in healthcare that protects the privacy and confidentiality of patient information. When a nurse discusses a patient's prognosis in a public setting like an elevator, it breaches the patient's right to privacy and violates their confidentiality.