Focal length
The distance at which a telescope's lens or mirror brings light to a focus; with the eyepiece it sets the magnification.
A telescope's focal length divided by an eyepiece's gives the magnification: 1,200 mm with a 25 mm eyepiece makes 48×. Long-focus telescopes (SCTs, Maksutovs) give high power easily; short-focus ones (fast Newtonians, small refractors) give wide, low-power views.
Barlow lenses and coma correctors lengthen the effective focal length; focal reducers shorten it.