Prime Lens
A lens with a single fixed focal length that cannot be zoomed.
Measured in
mm
In depth
With no need to hold performance across a range, a prime's designer can optimise for one focal length, which typically buys a wider maximum aperture, better correction, fewer elements and lower weight for the same money. The discipline of a fixed frame is also its own argument: moving your feet rather than a ring produces different, usually more considered, pictures.
Examples in practice
A 35 mm f/1.4 prime weighs less and shoots two stops wider than the 24-70 mm f/2.8 zoom that covers it.