A placeholder term used in science to describe an element which would be perfect for a specific use and is theoretically possible but beyond the current scope of human innovation, rare, or prohibitively expensive. In science fiction the term is often a trope used to describe "whatever is used to further the plot with the least amount of imagination and understanding of physics possible."
"In creating a design for a pulley system, our first challenge is to identify our unobtainium. In this case, the optimum material would lack both mass and friction."
PS I don't plan on doing pop culture references every week. Just this one. :-)