The Nikon F-mount is a type of interchangeable lens mount developed by Nikon for its 35 mm Single-lens reflex cameras. The F-mount was first introduced on the Nikon F camera in 1959, and features a three lug bayonet mount with a 44 mm throat and a flange to focal plane distance of 46.5 mm. At that time, the name of SLR cameras often included the letter R referring to Reflex (reflection), though the letter R sounds different phonetically in areas other than the English-speaking sphere; then, F was selected from Re-Flex for naming the Nikon F since the letter F is pronounced almost the same.
If you're a fan of the Nikon F-series SLRs and want to take a look inside one of your favorites cameras, then here are some cutaway pictures of the Nikon F-series SLRs - Nikon F, Nikon F2 and Nikon F3.
(Images via db)