What is the definition of aperture value?

I can responsibly tell you that the statement in the photography textbook is correct.

According to geometric optics: the aperture value is equal to the reciprocal of the relative aperture.

The lens pupil refers to the hole in the camera lens, between two optical lenses, whose diameter can be changed. In geometric optics, this hole is called the aperture diaphragm. Since the diameter of this hole will change, when we take pictures with a camera, we will choose different aperture values ??according to the needs of photography.