What does the "depth" mean on earth?

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I am really confused about the meaning of "depth" which is acquired by a RGB-D camera. I think there are two meanings,

1: The distance from a point in the 3-D world to the infrared camera center.

2: The Z value of the 3-D world point in the camera frame.

Which is right?

If the second meaning is right, the depth values of all the points in a wall which is parallel with the RGB-D camera should be equal, but the fact is not like that. However, many 3-D vision books indicate the second meaning is right. I am really confused. Who can help me?