Neither "Single stack" nor ".45 ACP" are good criteria as several companies make 1911s that defy both of those, most notably STI as they make a lot of models that are double stack and/or 9mm.
The easiest way to identify one imo is the familiar pattern of the pistol. All steel frame is a decent indicator but a few companies are making polymer frames now.
The best way to identify one internally, after seeing the outisde, is the swinging link locking mechanism combined with barrel lugs, 1911s do not use a linkless cam like most modern handguns.
I have seen the double stack ones like STI get called 2011s , not 1911s ...
I don't know too much about 1911's so here's my question: What are the distinct characteristics a gun must have to be considered a 1911?
I dunno.
I quite literally have lost count of the 1911s that I own many years ago. In just one brand alone I own many dozens of one variety of that brand. Yet virtually every 1911 that I own is different than every other.
TRADITIONALLY a 1911 would be a single stack magazine, a frame mounted safety that naturally falls under your right thumb, a magazine release button that situated next to the base of the trigger guard, a single action trigger, a grip safety, a 5" barrel with bushing.
For those of you wondering, Sig's 1911's really caught my attention. I'm trying to decide between the Scorpion, TacOps, and Nightmare. Though the MAX is very nice indeed.... Decisions, decisions!
Well, if the colt 1911 patents were still good, the 2011 would be in violation of almost every single patent.
If the 1911 patents were still good, the 2011 would probably still be entitled to the three that it has of its own.