What does .38 caliber mean?
Caliber is the diameter of the bore of a firearm. It's expressed in hundredths or thousandths of an inch, as in .45 caliber or .380 caliber. Curiously, the actual diameter of a .38 caliber slug is 0.357 inch.
When people talk of a 38, they mean .38 caliber, or 38/100th of an inch. Likewise, a 45 is .45 caliber or 45/100th of an inch. 45 > 38 so a 45 has a larger diameter. However, there are different lengths for each of those calibers, so a .38 Special is longer than a .45 ACP so in THAT sense, a 38 is larger (longer) than a 45.
.38 caliber firearms have been around for a long time- several muzzleloader, and cap & ball revolvers used that caliber (Colt Navy models). The .38 rimfire was created about 1861, and was followed by the .38 Webley, various .38 Colts, S&Ws, etc. The most common .38 cartridge today is the .38 Special (which is actually .357, not .38) It was created by Smith & Wesson in 1899, and is still in production.