A more involved question than it appears at first blush.
"
What makes a '1911' a '1911'?"
The Pistol, Model of 1911, as
cerberus stated, was adopted by the U. S. Government that year. It is a very specific design, developed by John Browning in cooperation with Colt's Firearms Company. Once adopted by the government, contracts were let to other companies to build it, and a relatively small number were produced by the original Springfield Armory.
In the original form, the 1911 was a recoil operated arm, with locking accomplished by barrel ribs mating with cuts inside the slide. Unlocking took place by means of a link pinned to the frame pulling the rear of the barrel downward as the locked barrel/slide moving rearward.
Standard barrel length: five inches
Ammunition supply: detachable seven round magazine, retained by a transverse catch located rear of the trigger..
Firing mechanism: Sliding trigger (non-pivoting) trips exposed hammer, to impact an inertial firing pin.
Thumb safety at left rear of frame.
Grip safety pivoted on thumb safety axle.
External slide lock above trigger; engages automatically when magazine is empty.
The model 1911 was updated in 1924 to the model 1911A1. This mainly entailed clearance cuts in the frame behind the trigger, longer grip safety tang, shorter hammer spur, making the flat mainspring housing into an arched configuration, and better sights. The 1911A1 series is still recognizable as the 1911. The Colt Commander, a shorter (4-1/4 inch barrel) version with aluminum alloy frame was introduced in 1949. While distinct from the "Government Model," it is still part of the 1911 family. Even shorter and lighter versions were introduced into the Colt's commercial line, direct descendants of the original 1911, but with limited arts interchange capability.
Originally, all 1911 pistols were in caliber .45. Later on, the basic design has been chambered for various other cartridges. The Colt Ace model was a .22 rim fire version, using the basic 1911 frame, with different slide, barrel, and magazine, and without the lockup system.
DragonFire, you wrote - -
The original design has been changed over the years, and lots of different manufacturers offer their own models. Lots of similarities and lots of differences. So makes them all a 1911?
Clearly, not all semi automatic .45 pistols are 1911s. Some versions are direct "clones" of the original, and others are distinctly different designs. I'd say that only the versions with a high degree of interchangeability with the original may be considered "1911s." These would include the Argentinean Sistema 1927 and the Norwegian 1914 models. Others, such as the Star, Astra, Llama, and Obregon designs, while paying homage to the 1911 inspiration, cannot be considered such.
Only part of the entire story - - -
Best,
Johnny