Technical term in photography and optics for the ratio of the lens's focal length to the diameter of the entrance pupil. Used when discussing aperture settings.
The f-number of this lens can be changed from f/2.8 to f/22.
When you make the f-number smaller, the background becomes more easily blurred.
Compound of 絞り (shibori, 'aperture, stopping down') and 値 (chi, 'value, numerical value'). A straightforward technical coinage.