1. Of an aircraft, to go into a sudden and rapid descent toward the ground leading with the nose of the plane. Everyone went into a panic when we started taking a nosedive, but the skilled pilot managed to regain control of the plane and landed it safely.

2. Of a person, to fall toward the ground face first. I was so distracted by the girl I like in class that I tripped and took a nosedive right in front of her.

3. By extension, to suddenly and rapidly begin to decline in physical, emotional, or psychological health or stability; to suffer a sudden loss or decline in value or success. My father has held out bravely in his fight against cancer, but I'm afraid he's taken a nosedive in the last couple of weeks. Shares in the company are taking a nosedive ever since news of the tax scandal went public.