First, let me come right out and say I have yet to see Avatar. However, having read a number of reviews and articles (not to mention being bombarded with ads on the FOX network) on Avatar, I am dismayed and very disappointed that James Cameron was so unoriginal that he actually used the term 'unobtainium' as the fictional mineral the humans are mining on the fictional planet Pandora.
You see, unobtainium is not a real element, and isn't meant to be. The term 'unobtainium' is a humorous term used by engineers to describe a metal (or other material) that is either so rare or costly it can't be practically used (e.g. antimatter), or is physically impossible. It is used to describe a material that is perfect for a certain project in every way, except for the small fact that it doesn't exist. Two classic examples of unobtainium are matter that is unaffected by gravity (i.e. massless), and matter that is unaffected by conventional matter (i.e. frictionless). Basically, engineers use the term 'unobtainium' to describe a material that is so advanced it is, by definition, unobtainable.
There are a few variations of that definition, but for the most part, they all describe the same thing: Something that is virtually, practically, or literally unobtainable.
The concept of unobtainium has been in use in science fiction probably since the very beginning. Science fiction writers are often faced with the fact that what they envision isn't possible given what we know of physics, so they turn to unobtainium.
In fact, the use of a material that doesn't exist (and probably can't) is a long-standing tradition in science fiction. The Warp Drives in Star Trek use a crystal called dilithium that is able to contain and channel a matter-antimatter explosion. Star Fleet vessels also use a metal known as tritainium for their hulls. Larry Niven's Ringworld is constructed of a material known as scrith, who's tensile strength rivals that of the Strong Nuclear Force. Marvel Comics' Woverine has a skeleton coated with adamantium, which is described as 'virtually indestructible'. All of these are examples of unobtainium.
However, the common thread all those examples have is a unique name. We, the audience, know these materials don't exist and are probably completely made up, and we don't care. But by giving these materials their own unique name, the writers are making their own universes unique. When I hear the word 'dilithium', I immediately think Star Trek. When I hear the word 'adamantium', I immediately think Wolverine. However, when I hear the word 'unobtainium', I think Engineer's Joke.
In 2003, I saw the film The Core. As a sci-fi flick, it wasn't bad. However, when it got to the scene when they introduced the Virgil's engineer, The Core veered directly into B-movie territory by announcing the hull of the Virgil would be made from unobtainium.
I swear to you, I laughed for a solid two minutes. My wife, of course, didn't understand why I was laughing. In between guffaws, I said: "Because unobtainium is an engineering joke! It doesn't exist and, by definition, can't exist! I can't believe they didn't spend 5 minutes thinking of a different name!"
Sci-fi script writers will often insert [technical term] when they want the technical writers/consultants to come up with a fancy technical-sounding term. This was like the writer had put in [unobtainium] and they decided to run with it. Now, The Core was based on a book, and the book may have used the term, so I can understand if they just didn't know any better. Their use of the term, however, dropped the film from B+ to B- in my book.
Then I heard that James Cameron used the same term in his new film Avatar. This time, I didn't find it so funny. I was really more disappointed and sad more than anything else. Apparently, I was the only one who thought it was hilarious when The Core tried to take the term seriously. I have always enjoyed James Cameron's films, and I have much respect for him as a director and writer, but I am surprised that he did not sit down for 5 minutes and think about a name for his mythical mineral. I am surprised that after spending $237 million, he went with a generic term, one that was never meant to be taken seriously.
Dear Mr. Cameron: Was 'pandorite' too trite for you? Was 'centaurium' too obvious? I thought of those in 20 seconds. Surely you could have done better, and I can't believe you didn't know better than to use the word 'unobtainium'. Now I'm afraid I just can't take your movie as seriously as I would have liked to.
Unobtainium is a joke, and by using it, I'm afraid your movie has become a joke as well.