If you read and understood the previous posts on this topic you should have a much better idea of what you wanted to know. Let me approach it just a little differently, though.
TECHNICALLY, caliber refers to the diameter of the bullet fired from a cartridge, whether it's measured in inches (US) or millimeters (European).
REALISTICALLY, caliber refers to the name of the cartridge itself, but it is expressed in a numeral value, often with an additional name added to that.
Just like the English language itself, there are a lot of inconsistencies in the nomenclature of guns and ammunition. There are American terms, British terms, European terms, and even archaic terms. It gets a little complicated sometimes, but be not dismayed!
.22 caliber can mean a diameter of .222" or .223" or even .224". In the real world, however, ".22 caliber" usually means a small .22 caliber rimfire cartridge (this refers to its ignition system with the priming compound in the crushable rim of the cartridge case). The commonly available .22 rimfires are called the .22 Short (not only the first .22 rimfire cartridge [1858?], but the first entirely self contained metallic cartridge OF ANY KIND), the .22 Long Rifle (originally created for use in rifles, but now equally at home in pistols), and the longer and more powerful .22 Magnum (NOT useable in guns made to shoot the .22 Short or .22 Long Rifle). See what I meant about the caliber really meaning the entire cartridge?
Old cartridges that originated back a hundred years ago or more combine the diameter (the appoximate diameter really) of the bullet with the amount of powder used to propel it measured in grains, a unit of weight, 7,200 of which make a pound if I remember right. Take the old .44-40 for example. It used a .44 caliber bullet that actually measured about .427" or .428" propelled by about 40 grains of the old style black powder.
An old cartridge that's one of my favorites is the .45-70, a real "stomper" if there ever was one. Because it was the official cartridge of the US Army back when Custer roamed the plains, it was officially called the .45-70 Govt. And to further complicate things, it has been loaded using bullets of different weights, hence the designations .45-70-405 (using a 405 grain bullet over 70 grains of powder) and .45-70-500, with or without the Govt. tag hung onto it.
A somewhat more modern cartridge that's another one of my favorites was Sweden's official rifle cartridge for a hundred years, the 6.5 x 55, or more correctly the 6.5 mm by 55 mm, given in metric terms. This cartridge is nearly as modern today as when it was first introduced to the Swedish army in about 1894. (I think it's been superceded by the 7.62 mm NATO round now, but that's beside the point.) The 6.5 x 55 Swedish uses a bullet of 6.5 mm diameter, which is .264" in US terms. The length of the case, 55 mm, is used in the nomenclature of this cartridge. This is an extremely efficient and lethal cartridge that rivals most of today's so-called "modern" rounds. It is easy on the shooter, extremely accurate, and very, very effective. If you EVER find one of the old surplus Swedish Mauser rifles in decent shape, BUY IT. You won't be sorry, I promise you that. Quite a few came into this country at different times, including a lot of them about ten years ago, so there are still a lot of them around. You won't find them for $100 any more, though. Try $250. They do turn up sometimes, though, and you may luck out and get one for about $150 in a pawn shop or somewhere like that. I have.
But you asked what time it is and I told you how to build a watch.
