Focal ratio
Also called: f-number, f-ratio, f/, fast, slow.
Focal length divided by aperture (f/5, f/10…). “Fast” (low) ratios give wide fields; “slow” (high) ones give easy high power.
An f/5 telescope has a focal length five times its aperture. For visual use, the focal ratio sets how wide a field your eyepieces can give and how short an eyepiece you need for high power — exit pupil in mm is the eyepiece's focal length divided by the focal ratio.
Fast Newtonians (below about f/5.5) suffer from coma at the edge of the field and need precise collimation; slow ones are forgiving.