Technical term used in photography and optics to describe the ratio of the lens's focal length to the diameter of the entrance pupil. Often seen in specifications and discussions about aperture.
The f-number of this lens is F2.8.
The smaller the f-number, the easier it is to blur the background.
Compound of 開放 (かいほう, 'open, aperture') and 値 (ち, 'value, numerical value'). The term directly translates to 'aperture value'.