We've become accustomed to referring to the Saturn vehicles as the "Saturn I", "Saturn IB", and "Saturn V". However, for the several years these rockets were referred to instead as the "Saturn C-1", "Saturn C-1B", and "Saturn C-5" (actually, the Saturn I was known as "Juno V" for the first 5 ½ months or so of its life -- from August 15, 1958 until February 2, 1959 -- but that's a different story).
There's conflicting information as to what the "C" in "C-1/1B/5" actually stood for. You sometimes see it defined as "configuration" or "concept". Early design committees came up with various families of Saturn configurations (there's that "C"-word again!), including an "initial" configuration, a "B" family of configuration (including vehicles such as the B-1, B-2, B-3, and B-4) and a "C" configuration. It seems likely that there were people working on the program who thought it meant "configuration" while others thought it was "concept", while still others "knew" it was the third family of configurations. So, take your pick.
NASA News Release No. 63-26, issued on February 7, 1963, renamed the Saturn "C-" series names to the more familiar I/IB/V series.
NASA Simplifies Names of Saturn Launch Vehicles
The National Aeronautics and Space Administration today announced slight changes in the names of Saturn launch vehicles in the interest of simplification.
The original member of the family of three Saturn vehicles, known from its conception as Saturn C-1, will hereafter be called "Saturn I." The Saturn C-IB will now be known as "Saturn IB," and the Saturn C-5, (often called the Advanced Saturn) becomes "Saturn V."
These rockets are being developed for NASA's Office of Manned Space Flight under the direction of the NASA Marshall Space Flight Center, Huntsville, Ala. All three will be used in the Apollo Manned Lunar exploration program.
The Saturn I will be used to place the command module of the Apollo spacecraft, bearing a three-man crew, into earth orbit for up to two weeks. The Saturn I has a first stage (called S-I) which is 21½ feet in diameter, 80 feet in length and generating 1.5 million pounds thrust from eight H-1 engines. The rocket's second and final stage will be the S-IV, 18 feet in diameter, 41 feet in length and generating 90,000 pounds thrust from six RL-10 engines.
The second vehicle, Saturn IB, will likewise be used in early development and training phases of project Apollo. It will be capable of placing the complete Apollo spacecraft into earth orbit. This vehicle consists of the same first stage as used on the Saturn I, plus a different second stage, the S-IVB. The S-IVB will be 21½ feet in diameter, about 58 feet in length, and will be powered by a J-2 engine developing 200,000 pounds of thrust. The Saturn IB will be capable of placing 16 tons into earth orbit, compared to 10 tons for Saturn I.
The largest of the three vehicles, the Saturn V, is the Apollo moon rocket. It consists of a first stage (S-IC) 33 feet in diameter, 138 feet long and powered bv five F-1 engines generating 7.5 million pounds thrust; a second stage (S-II) 33 feet in diameter, 82 feet long and developing 1 million pounds thrust from five J-2 engines; and a third stage which is the same S-IVB stage used on the Saturn IB. This rocket will be able to place about 120 tons into earth orbit and send 45 tons to the vicinity of the moon. It will be used for manned lunar landings.