f stop

Short definition from Wiktionary

Noun

1. 1
   Alternative spelling of f-stop.
2. 2
   Any of the discrete steps or stopping points for adjusting the aperture of a lens, either marked on a ring on the lens and adjusted by rotating that ring or marked in the display of a digital camera and adjusted by buttons or touch-sensitive controls.
3. 3
   Alternative spelling of f-stop.