I'm not aware of a film-oriented version of the term "unobtainium", but for engineers and scientists, unobtainium is something that you need but is not obtainable...  hence "unobtainium". It would be something like cold fusion or a room-temperature superconductor or matter older than the universe, etc. Something that would solve your problem perfectly, if only it existed.

1) The stuff in Avatar is not technically "unobtainium" in the sense that it's...  you know... obtainable. However, the people in the film are referring to it by name as unobtainium, so I'm assuming that they named this stuff unobtainium because they had previously considered it somehow impossible. Then they discovered this stuff which defied their logic and solved some hypothetical engineering problem somehow. I assumed while watching the film that they named it after the engineering term for this reason. They don't really go into what exactly its properties are, but it's worth a buttload of money, so it's apparently something really awesome that everyone wants but which there's not nearly enough of to go around. I think for the plot that's about all you need to know.

2) I do consider it a MacGuffin. It's something the humans want and gives them sufficient motivation to drive their actions. They do want the money, but the money doesn't fund the Avatar program. At least that's not all it funds. One must assume that there are some extremely wealthy CEO's on earth, not to mention an industry of some sort that relies on this mineral. The Avatar program is funded by the company in an effort to make the acquisition of unobtainium easier. The mineral itself seems to be worth a lot more than the avatars, guessing from the sorta cavalier attitude the military guys have toward them at the end of the film.

Actually, what I found funny about the way they dealt with the stuff is that Ribisi's character is all "This shit is worth hella bucks, dawg! Scope out the size of that deposit! We need a piece of that shit!" So...  by selling a buttload of the stuff, demand would go down and the price would go down, thus hurting their business. Maybe their plan was to sit on the stuff like the diamond industry...

GUH.

Hate.

They do, tho. Lowering oil production causes the price per barrel to skyrocket.

Zarban always makes me feel better.

never did i say that I was treating it as canon. (If you'll note I both bookended both sides of that sentence with "according to Cameron").

And honestly I was just making an off hand statement/joke.

FTFY

Interstellar travel is orders of magnitude more difficult that simply sailing or flying across the ocean. Surely you've seen those scaling demonstrations that highlight the vast distances between stars. Then you factor in the speed of light limit, and no one is going anywhere.

To land a man on nearby Mars for a few months by 2050 would cost $1 trillion, and no country is even thinking about it. Imagine the cost of sending mining equipment and sending back ore. It'd cost more than the combined GDP of Earth. Far simpler just to make it ourselves. All stuff is just atoms. Each proton is the same as any other proton.

And that's just the local neighborhood, let alone another star system which would take hundreds of years to get to and back from.

CERN, Fermilab, Unilac, etc can turn one element into another by neutron bombardment. Fission and fusion are 'alchemical' processes in that they transmute elements. Since the quantum revolution of the 1920s, we know that adding or subtracting protons can change an element. In fact, the very definition of an element is solely dependent on the number of protons and nothing else.

We can even make elements that don't exist in nature. One facility in Germany specialises in that. Natural elements only occur up to 92 (Uranium), but we can make elements up to 118 (and rising).

The point is we knew how to do this in the 1940s, but it won't be for hundreds of years until we can reach other stars. If anything, we're going backwards now that the Shuttle is retired.

And if "unobtanium" is a mineral, we'll that's even easier to produce than a new element, as minerals are simply a function of temperature pressure, and composition.

So I think the conclusion is valid: travelling to other stars is vastly more difficult than producing a mineral or an element. There'll be scientific reasons for interstellar travel (especially with probes), but not for mining ore. Avatar is just commentary on the 16th century Spanish conquest of the Americas, and all other subsequent colonisations. The dynamics of Earth's geopolitics have just been lazily transposed to space, but without allowing for the fact that space significantly alters the cost/benefit equation.

I realize that the notion of mining simple minerals from other star systems is, on the face of it, ridiculous. I got the impression that the comically-named Unobtanium was, in some way, magical and therefore, by extension, not readily synthesizable in other locations. Floaty-mountains, trees-of-life, et cetera.

But this discussion of the economics of interstellar mining reminds me of the, let's call them, debates that I saw after Star Trek 2009 came out, and various self-declared experts traded verbal blows on the subject of whether starships ought to be built on the ground, or in space. As if Starfleet Engineering were a real thing, and we had the first clue how to go about it.

To me, it's like sixteenth-century entrepreneurs discussing the merits of creating Facebook.

I presumed 'magic', because the movie features floating mountains in an area where gravity still works the way it does elsewhere in the vicinity; waterfalls fall down, there aren't huge winds caused by differing air masses, etc.. Already, whatever the laws of physics are for us, they're different in the movie. Maybe those mountains don't have a mass of a million tons, but only a million grams.

Plus, not wishing to sound like a Baroquen web developer, but what happens to the mass-times-distance economic argument when some bugger invents warp drive, and we get to short-cut 99.9999% of the way?