What are defocus, bokeh, and exposure reciprocity?

Defocus: the light does not converge at the focus, but diverges;

Bokeh: the blurry area outside the focus;

Exposure reciprocity: the same Exposure can have different combinations of aperture, shutter, and ISO. As long as the total amount of light entering remains unchanged, there can be many combinations of aperture, shutter, and ISO.