Prime lens

in film and photography, a prime lens is a fixed focal length photographic lens , typically with a maximum aperture from f2.8 to f1.2. The term can also mean the primary lens in a combination lens system. Confusion between these two meanings can occur if context doesn't make the interpretation clear. People sometimes use alternate terms primary focal length, fixed focal length.

For those of you unfamiliar with the term ‘prime lens’ lets start with a simple definition. A prime lens is one that has just one focal length only (in contrast to a zoom lens that covers a wider range of lengths).
Prime lenses come in a wide range of focal lengths from wide angles through to the very longest of tele-photo lenses used by many sports photographers and paparazzi.
While zoom lenses are ever popular and come as the standard kit lens with most DSLRs sold – prime lenses remain the favorite of many.
These days, most lenses in circulation have some kind of zoom capability. To zoom, you either twist the lens or press some buttons on the back panel of the camera. The camera then adjusts the focal length of its lens, allowing you to see things that are further away with better clarity.
Prime lenses are different. They only have one focal length. You can’t zoom in or out. You get one viewing distance, and that’s it.
The most standard prime lens is the 50mm “normal lens.” Its focal length roughly matches where your eye focuses, giving you an image that’s fairly close to what’s in front of you. There are other prime lenses too that come in all sorts of different focal lengths.