I quite like the idea from Michael and Stephen that it's the lead screw that makes it an auger rather than a drill bit. Except what about the bull nose Single Twist or L'Hommedieu variety with no lead screw?
Wadda you mean "The what?" You don't all have them in assorted sizes...? I do. (Auger problem? What auger problem...?) But unfortunately I have a slippery slope of the icy rather than the toolish variety twixt me and a photograph of same. But trust me, very definitely augers without a lead screw. Favoured by shipwrights, I gather. (Ah, Darrell LaRue's page has some here. Couldn't find that page when I looked before; thought it had died. Does the world need another brace bit reference? Probably not. That'll save me some work then...)
Howard wisely zeroes in on the spiral or helical screw of the auger, but as we know, the average person asked to point out a drill bit would pick out something also with a spiral cutting edge. An email correspondent takes it further though, and I think this may be the closest we'll get to a rule that'll stick. Viz:
"We typically call any drill bit where the helical vein is less than the root diameter a "drill bit" and any drill bit where the helical vein is larger than the root diameter an "auger bit.""
Bend your brain to that one, gentle reader, and see how you like it. It doesn't cover every eventuality either really, but I think the essential problem with trying to define the difference is the initial labelling is rather wayward. We know from custom what we'd call an auger and what we'd call a drill bit, but how often does custom come into being based on a precise and technical definition of something? Yeah, exactly. We may be on a hiding to nothing here.
Happily though, we have side benefits and discussion as a result. Such vital questions as "Why do catalogues list braces and not the bits to use in them?" and "Whither the forstner bit?" These are good points.
The forstner bit for braces, I have no idea about. Where are they? I've never seen a single one. Not one. Not even my encounter with the patternmaker's tools lead to finding any, and that was as likely a source as any. Like 12" and 14" braces, were they just something listed in catalogues but really only Americans ever bought them? I have no idea. Of course there are modern ones with round shanks, which leads us to question two...
Modern braces, but no brace bits. I confess I was a little horrified to hear this. Last I checked - and in fairness, it was a while ago - Axminster, for one, listed Clico Jennings Pattern auger bits. But not any more. Lee Valley has a few square-tanged brace bits, such as these Spoon Bits, but it is indeed a bit of a desert out there for new bits. Having said which, do not modern braces tend to be designed to take hexagonal and even round shanks? I know certain older models of brace chuck were also able to do that, and indeed a basic two-jaw brace will generally cope with hex-shanks pretty well in my experience. So you could argue they are listing bits that'll fit the braces they sell, they're just not what any right-thinking neanderthal would consider the bits of choice.
Personally I think a serious revival of hand boring methods is well overdue and we should all jump on the band wagon in a hurry before The Schwarz finishes inadvertently putting the price of admission completely out of our reach. Although I'm still kinda hoping he'll discover the joys of a fluted drill bit in a hand drill so someone'll put them back in production. In the meantime, I fear it's to the car boot, the flea market, and the dreaded online auction site if we wish to feed out brace bit habit.
Which we haven't got. Well I haven't; can't speak for you...