Which was the first movie that used the word "Unobtainium" to identify an element?
Unobtainium can refer to any substance needed to build some device critical to the plot of a science fiction story, but which does not exist in the universe as we know it. A hull material that gets stronger with pressure in the film The Core (2003) was nicknamed unobtainium, but the concept under different names can be seen in the anti-gravity material cavorite from H. G. Wells' 1901 novel The First Men in the Moon, as well as the super-strong material scrith from Larry Niven's novel Ringworld, which requires a tensile strength on the order of the forces binding an atomic nucleus.
The term was used in James Cameron's 2009 movie Avatar, as a substance that was named (in the film's dialog) unobtanium (note the slightly different spelling). In the film, it was mined on the fictional moon Pandora, and was a room-temperature superconductor.