The name Summicron is used by Leica to designate camera lenses that have a maximum aperture of f/2 after 1953 and to present day.
Leica designed a number of f/2 lenses before the Summicron, such as the Summar and Summitar. New coating technologies available after World War II allowed for less distortion and better aberration management, leading to the Summicron. The first Summicron was an evolved Summitar collapsible 50mm with new Lanthanum glass, and was launched in 1953. It was available in M39 mount ("screw mount"), then made available in M-mount for the Leica M3.
Faster Leica lenses are offered with the trade names Nocticron or Noctilux. Elmarit lenses are slower.