M42 (also known as Pentax Universal) is a screw-type lens mount for SLR cameras. It was originally developed by Carl Zeiss but was adopted by many other manufacturers throughout Europe and Asia. Other notable M42 manufacturers include Praktica, Fujica, Pentax, and Zenit.
M42 is a very simple system with a long flange distance, making adapters simple and common. Many film manufacturers made adapters for mounting M42 lenses to their systems, and they can also be used with modern SLR & mirrorless cameras quite easily. Because it was used by so many companies, there are a huge number of M42 lenses out there, covering every focal length and level of optical quality.